Fixed string Join method
authorDan Moseley <danmose@microsoft.com>
Wed, 16 Nov 2016 20:55:45 +0000 (12:55 -0800)
committerGitHub <noreply@github.com>
Wed, 16 Nov 2016 20:55:45 +0000 (12:55 -0800)
Fixed string Join(string separator, params object[] values) method.
Calling string.Join(",", null, 1, 2, 3); return empty string but should ",1,2,3".

Commit migrated from https://github.com/dotnet/coreclr/commit/b6e5a7fd2deaf73cb45ad82474b9e4beee7883a4


Trivial merge