Fix a crash where Catch was dereferencing a NULL string.

This line was crashing:

        REQUIRE(testObject.concreteProperty == (id)nil);

where `concreteProperty` is a property of type `NSString*`. I discovered that CATCH was choosing the `toString(NSString *nsstring)` overload rather than the `toString(nullptr_t)` overload. The crash was in `std::string::operator+()` trying to find the length of the NULL string `[NSString UTF8String]`.

I would have included a test demonstrating my change but it appears that SelfTest doesn't support ObjC++…