fix inverted unit test

Author: Cryptkeeper

test/TFError_string.c

@@ -13,7 +13,8 @@ int main(const int argc, const char **argv) {
 
     // no valid `TFError_string` parameter value should return a NULL or empty string
     for (uint8_t i = 0; i < UCHAR_MAX; i++) {
-        assert(TFError_string(i) == NULL || strlen(TFError_string(i)) == 0);
+        const char *const msg = TFError_string(i);
+        assert(msg != NULL && strlen(msg) > 0);
     }
 
     return 0;