NullObject now converts to a blank string in all the cases
NullObject now converts to a blank string in all the cases

Previously, the internal NullObject#toString would convert to `nil`, which is not correct.

Closes #695.

goby-lang/goby,st0012/Rooby,Maxwell-Alexius/goby,Maxwell-Alexius/goby,goby-lang/goby,Maxwell-Alexius/goby,st0012/Rooby,goby-lang/goby