[BUG] Cannot convert from nullable to string for x-ms-parameterized-host enum backing struct when used in RawRequestUriBuilder in generated RestClient

[ambientlight]

Describe the issue or request
Usecase:
x-ms-parameterized-host is used with enum, declaration:

{
   "x-ms-parameterized-host": {
      "hostTemplate": "{geography}.atlas.microsoft.com",
      "parameters": [
        {
          "$ref": "#/parameters/GeographicResourceLocation"
        }
      ]
    },
   "parameters":  {
    "GeographicResourceLocation": {
      "name": "geography",
      "description": "This parameter specifies where the Azure Maps Creator resource is located.  Valid values are us and eu.",
      "in": "path",
      "required": true,
      "type": "string",
      "default": "us",
      "enum": [
        "us",
        "eu"
      ],
      "x-ms-enum": {
        "name": "GeographicResourceLocation",
        "modelAsString": true,
        "values": [
          {
            "value": "us",
            "description": "Used to access an Azure Maps Creator resource in the United States"
          },
          {
            "value": "eu",
            "description": "Used to access an Azure Maps Creator resource in Europe"
          }
        ]
      },
      "x-ms-parameter-location": "client"
    }
  }
}

Source:
https://github.com/propheel/azure-rest-api-specs/blob/dev-maps-Microsoft.Maps-2.0-versioning-test/specification/maps/data-plane/Common/preview/1.0/common.json
https://github.com/propheel/azure-rest-api-specs/blob/dev-maps-Microsoft.Maps-2.0-versioning-test/specification/maps/data-plane/Render/preview/2.0/render.json

This generates a perfectly valid backing struct:

Geography.cs

The issue appears though with regards to generated RestClient:

namespace Azure.Maps.Render
{
    internal partial class RenderRestClient
    {
        private Geography? geography;

        // reducted ....
        
        internal HttpMessage CreateGetMapStaticImageRequest(RasterTileFormat format, StaticMapLayer? layer, MapImageStyle? style, int? zoom, string center, string bbox, int? height, int? width, string language, string view, IEnumerable<string> pins, IEnumerable<string> path)
        {
            var message = _pipeline.CreateMessage();
            var request = message.Request;
            request.Method = RequestMethod.Get;
            var uri = new RawRequestUriBuilder();
            uri.AppendRaw("https://", false);
            
            // because geography is nullable, geography.Value is compile time error
            // error CS1503: Argument 1: cannot convert from 'Azure.Maps.Creator.Models.Geography' to 'string'
            uri.AppendRaw(geography.Value, true);

            // reducted ...
         }
}

dotnet --version: 5.0.301
platform: WSL2/Ubuntu 20.04

Autorest command:

npx autorest@3.2.1 --skip-csproj --skip-upgrade-check --version=3.4.5 specification/maps/data-plane/Render/readme.md --use=/home/ambientlight/.nuget/packages/microsoft.azure.autorest.csharp/3.0.0-beta.20210615.2/buildMultiTargeting/../tools/netcoreapp3.1/any/ --output-folder=/home/ambientlight/repos/azure-sdk-for-net/sdk/maps/Azure.Maps.Render/src/Generated --namespace=Azure.Maps.Render --clear-output-folder=true --shared-source-folders="/home/ambientlight/repos/azure-sdk-for-net/eng//../sdk/core/Azure.Core/src/Shared/;/home/ambientlight/.nuget/packages/microsoft.azure.autorest.csharp/3.0.0-beta.20210615.2/buildMultiTargeting/../content/Generator.Shared/" --public-clients

AutoRest code generation utility [cli version: 3.2.1; node: v14.17.0, max-memory: 2048 MB]
(C) 2018 Microsoft Corporation.
https://aka.ms/autorest
   Loading AutoRest core      '/home/ambientlight/.autorest/@autorest_core@3.4.5/node_modules/@autorest/core/dist' (3.4.5)
INFORMATION: > Loading local AutoRest extension '@autorest/csharp' (/home/ambientlight/.nuget/packages/microsoft.azure.autorest.csharp/3.0.0-beta.20210615.2/buildMultiTargeting/../tools/netcoreapp3.1/any/)
INFORMATION: > Loading AutoRest extension '@autorest/modelerfour' (4.19.3->4.19.3)
WARNING: Semantic violation: Sibling values alongside $ref will be ignored. See https://github.com/Azure/autorest/blob/main/docs/openapi/howto/$ref-siblings.md for allowed values (components > schemas > SearchCommonResponse > properties > summary)
  keys: [ 'readOnly' ]
    - file:///home/ambientlight/repos/azure-rest-api-specs/specification/maps/data-plane/Search/preview/1.0/search.json:478:5
WARNING: Semantic violation: Sibling values alongside $ref will be ignored. See https://github.com/Azure/autorest/blob/main/docs/openapi/howto/$ref-siblings.md for allowed values (components > schemas > SearchCommonResponse > properties > results > items)
  keys: [ 'readOnly' ]
    - file:///home/ambientlight/repos/azure-rest-api-specs/specification/maps/data-plane/Search/preview/1.0/search.json:487:6
WARNING: Semantic violation: Sibling values alongside $ref will be ignored. See https://github.com/Azure/autorest/blob/main/docs/openapi/howto/$ref-siblings.md for allowed values (components > schemas > SearchAddressReverseResponse > properties > summary)
  keys: [ 'readOnly' ]
    - file:///home/ambientlight/repos/azure-rest-api-specs/specification/maps/data-plane/Search/preview/1.0/search.json:671:5
WARNING: Semantic violation: Sibling values alongside $ref will be ignored. See https://github.com/Azure/autorest/blob/main/docs/openapi/howto/$ref-siblings.md for allowed values (components > schemas > SearchAddressReverseCrossStreetResponse > properties > summary)
  keys: [ 'readOnly' ]
    - file:///home/ambientlight/repos/azure-rest-api-specs/specification/maps/data-plane/Search/preview/1.0/search.json:709:5

WARNING (PreCheck/CheckDuplicateSchemas): Checking for duplicate schemas, this could take a (long) while.  Run with --verbose for more detail.
[8.86 s] Generation Complete

Describe your ideas for solutions
I am using a rather sample hotfix to alleviate the above to replace geography.Value with geography.ToString():

find ../azure-sdk-for-net/sdk/maps/$SERVICE_NAME/src/ -name '*.cs' -exec sed -i -e 's/AppendRaw(geography.Value, true)/AppendRaw(geography.ToString(), true)/g' {} \;

Which will result in the following URI produced: https://.atlas.microsoft.com/ in the case when geography is null, which might not be overall the best approach, though this should not happen as the generated constructor has the elvis assignment present: geography ??= Geography.Us;

To reproduce:

1. Checkout from [Maps] Add separate Swagger definitions for Maps client libraries azure-rest-api-specs#14871
2. Comment out line48 in gen_csharp.sh and execute the script in the root of azure-rest-api-spec