Fixed erroneous operator== on ZString.

testing/zstring_test.cpp

@@ -105,5 +105,7 @@ int main(int argc, char **argv) {
 
    std::cout << "Null assignment" << std::endl;
    coreutils::ZString test20(NULL);
+   std::cout << "[" << test20 << "]" << std::endl;
+   std::cout << "Comparisons with null value: " << (test20 == "true") << std::endl;
    
 }