Account for missing contact in dfrn_request_post

- Address https://github.com/friendica/friendica/issues/9250#issuecomment-743407998

mod/dfrn_request.php

@@ -299,7 +299,7 @@ function dfrn_request_post(App $a)
 		$network = $data["network"];
 
 		// Canonicalize email-style profile locator
-		$url = Probe::webfingerDfrn($data['url'], $hcard);
+		$url = Probe::webfingerDfrn($data['url'] ?? $url, $hcard);
 
 		if (substr($url, 0, 5) === 'stat:') {
 			// Every time we detect the remote subscription we define this as OStatus.

src/Network/Probe.php

@@ -258,7 +258,7 @@ class Probe
 	 * @return string profile link
 	 * @throws HTTPException\InternalServerErrorException
 	 */
-	public static function webfingerDfrn($webbie, &$hcard_url)
+	public static function webfingerDfrn(string $webbie, string &$hcard_url)
 	{
 		$profile_link = '';