Short circuit in ToUrl to remove allocations when possible.

[stevejgordon]

In the process of going through the code, I spotted a fairly trivial change which can reduce allocations during URL generation for endpoints where no route values need to be applied.

UrlLookup.ToUrl can short-circuit in such cases to avoid duplicating the string and allocating a StringBuilder. For requests where no index is supplied, the allocations can be entirely removed and an 87% reduction in execution time can be achieved.

The examples below test with the cluster health endpoints.

Before:

|      Method |     Mean |    Error |   StdDev |  Gen 0 | Gen 1 | Gen 2 | Allocated |
|------------ |---------:|---------:|---------:|-------:|------:|------:|----------:|
|      Health | 41.60 ns | 0.637 ns | 0.596 ns | 0.0381 |     - |     - |     160 B |
| HealthIndex | 85.60 ns | 0.851 ns | 0.796 ns | 0.0457 |     - |     - |     192 B |

After:

|      Method |      Mean |     Error |    StdDev |  Gen 0 | Gen 1 | Gen 2 | Allocated |
|------------ |----------:|----------:|----------:|-------:|------:|------:|----------:|
|      Health |  5.352 ns | 0.0611 ns | 0.0510 ns |      - |     - |     - |         - |
| HealthIndex | 84.470 ns | 0.5005 ns | 0.4437 ns | 0.0457 |     - |     - |     192 B |

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[russcam]

LGTM, thanks @stevejgordon 👍