CosmosClientOptions and ClientBuilder: Fixes ArgumentException when setting null value on HttpClientFactory or WebProxy

[j82w]

Pull Request Template

Description

This fixes the following issues:

1. Previously WebProxy would get set even if an argument exception was thrown
2. Setting a null value caused an ArgumentException which is not necessary for null values.
3. WebProxy can now be set without being in gateway mode like HttpClientFactory. It's possible users want custom HttpClient settings even when in direct mode because control plane operation like Database and Container still go through HttpClient.

Type of change

Please delete options that are not relevant.

• [] Bug fix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [] This change requires a documentation update

Closing issues

closes #1953

[ghost]

Closing due to in-activity, pease feel free to re-open.