ParsingHelpers.GetHeaderSplit - Check if values is empty before parsing

[snakefoot]

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

HttpAbstractions/src/Microsoft.AspNetCore.Http.Abstractions/Internal/ParsingHelpers.cs

Lines 19 to 23 in ec176b2

	public static StringValues GetHeaderSplit(IHeaderDictionary headers, string key)
	{
	var values = GetHeaderUnmodified(headers, key);
	return new StringValues(GetHeaderSplitImplementation(values).ToArray());
	}

[snakefoot]

Continue at dotnet/aspnetcore#3737