Find the correct app's SSH URL when no instance or default is given

This uses the instance-based SSH URL if an app default URL is not available and
the $instance is not provided. So it avoids falling back to the environment's
default SSH URL in more cases.

src/Model/Environment.php

@@ -118,8 +118,8 @@ public function getHeadCommit()
     public function getSshUrl($app = '', $instance = '')
     {
         $urls = $this->getSshUrls();
+        $instances = $this->getSshInstanceURLs($app, $urls);
         if ($instance !== '' && $instance !== null) {
-            $instances = $this->getSshInstanceURLs($app, $urls);
             if (isset($instances[$instance])) {
                 return $instances[$instance];
             }
@@ -132,6 +132,9 @@ public function getSshUrl($app = '', $instance = '')
         if (isset($urls[$app])) {
             return $urls[$app];
         }
+        if (!empty($instances)) {
+            return reset($instances);
+        }
 
         // Fall back to the legacy SSH URL.
         return $this->constructLegacySshUrl();