Fixed == operator for empty string comparison.

testing/zstring_test.cpp

@@ -6,8 +6,9 @@ int main(int argc, char **argv) {
 
    // Test comparison operations.
 
-   coreutils::ZString test1("");
+   coreutils::ZString test1("else");
    std::cout << (test1 != ".") << std::endl;
+   std::cout << (test1 == "") << std::endl;
 
    // Test constructors.