ParsingHelpers.GetHeaderSplit - Check if values is empty before parsing

[snakefoot]

Not calling GetHeaderSplitImplementation will skip the allocation of a yield-state-machine:

https://github.com/aspnet/HttpAbstractions/blob/ec176b2e40699c24cc1c0e3bde8a828e93fe67c7/src/Microsoft.AspNetCore.Http.Abstractions/Internal/ParsingHelpers.cs#L19-L23

[Tratcher]

Note we avoid micro-optimizations unless we can demonstrate their impact on a benchmark.

[snakefoot]

@Tratcher What should the scope of the benchmark be?

Just calling the HeaderDictionaryExtensions.GetCommaSeparatedValues() on an empty IHeaderDictionary many times ?

[Tratcher]

You'd need the negative case as well, where GetCommaSeparatedValues was called with non-empty values.

The difference for such micro-benchmarks would need to be substantial in order to have any effect on the larger application.

[BrennanConroy]

Done via #9309