Eliminate allocations in AuthorizationPolicy.CombineAsync with no policy

[JamesNK]

Used by AuthorizationMiddleware. Want to avoid allocations when there is no policy to evaluate.

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On #1894

[davidfowl]

Where's the before and after number

[JamesNK]

Note: I added caching to policy provider tasks.

Before:

 Method | Mean | Error | StdDev | Op/s | Gen 0 | Allocated | ------------- |---------:|---------:|---------:|------------:|-------:|----------:| CombineAsync | 100.7 ns | 2.348 ns | 4.174 ns | 9,927,843.4 | 0.0203 | 184 B | 

After:

 Method | Mean | Error | StdDev | Op/s | Allocated | ------------- |---------:|----------:|----------:|-------------:|----------:| CombineAsync | 57.57 ns | 0.5720 ns | 0.5351 ns | 17,371,422.1 | 0 B | 

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