TimeSpan.TryParseExact should not try to parse null input.

TimeSpan.TryParseExact would previously throw a NullReferenceException constructing the Parser when it accesses the Length property of the input string. Microsoft's implementation returns false when null is passed as the input string to parse.
Merge pull request #653 from LogosBible/TimeSpan-TryParseExact

TimeSpan.TryParseExact should not try to parse null input.