ComposerRepository: fix array_keys(): Argument #1 () must be of type array, null given (#10529)

src/Composer/Repository/ComposerRepository.php

@@ -657,7 +657,7 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
             return array();
         }
 
-        if ($this->providersUrl) {
+        if (null !== $this->providersUrl && null !== $this->providerListing) {
             return array_keys($this->providerListing);
         }