tool_getparam: make --doh-url "" switch it off

A possible future addition could be to parse the URL first too to verify
that it is valid before trying to use it.

Assisted-by: Jay Satiro
Closes #9207

src/tool_getparam.c

@@ -701,6 +701,9 @@ ParameterError getparameter(const char *flag, /* f or -long-flag */
         break;
       case 'C': /* doh-url */
         GetStr(&config->doh_url, nextarg);
+        if(config->doh_url && !config->doh_url[0])
+          /* if given a blank string, we make it NULL again */
+          Curl_safefree(config->doh_url);
         break;
       case 'd': /* ciphers */
         GetStr(&config->cipher_list, nextarg);