Fixed Join(string separator, params object[] values) method

[AlexRadch]

Fixed Join(string separator, params object[] values) method.
Calling string.Join(",", null, 1, 2, 3); return empty string but should ",1,2,3".

[dnfclas]

Hi @AlexRadch, I'm your friendly neighborhood .NET Foundation Pull Request Bot (You can call me DNFBOT). Thanks for your contribution!

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[jkotas]

@AlexRadch Thanks for submitting the fix. We are doing the behavior changes like this one in coreclr/corefx first, and only then porting them to corert.

This issue is tracked as https://github.com/dotnet/coreclr/issues/4506. Check the discussion there - it is a tricky one because of this behavior is explicitly mentioned in the documentation.

[dnfclas]

@AlexRadch, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, DNFBOT;

[dnfclas]

@AlexRadch, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, DNFBOT;

[jkotas]

Matching CoreCLR issue was merged dotnet/coreclr#8114

[jkotas]

@AlexRadch Thanks!