GetCommaSeparatedValues should strip out empty strings

[JunTaoLuo]

We should ignore empty headers since they are not meaningful. Currently, parsing ,, will generate three empty strings {string[3]}: {"","",""}. We should instead check whether the parsed values are empty here: https://github.com/aspnet/HttpAbstractions/blob/dev/src/Microsoft.AspNetCore.Http.Abstractions/Internal/ParsingHelpers.cs#L29