No error when no test cases exist

This pull request resolves a "NoMethodError: undefined method `>' for nil:NilClass" error that would occur if no tests cases were defined.  The issue is in this line below, where @test_count will be nil if no tests exist.

http://github.com/seattlerb/minitest/blob/dcdb8042533697caae7933cab658468b4e13ae0d/lib/minitest/unit.rb#L1054

I doubt this happens often, but getting no result instead of a nil error might be easier for users.