Will now throw LibGit2SharpException with proper message when 'user.name...

...' or 'user.email' global setting is set to an empty value

When global user.name or user.email is set to an empty value, it will
now throw a LibGit2SharpException with proper explanatory message.
Previously it would throw an ArgumentException with obscure message:
"String cannot be empty\nParameter name: name".
We've already made sure that neither the name or email are `null` or empty. Shouldn't we get rid of the `null` handling here?