Don’t add github specials if there is no composer data

This can happen if an identifier doesn’t have a composer.json file (but other identifiers do)

src/Composer/Repository/Vcs/GitHubDriver.php

@@ -152,6 +152,7 @@ class GitHubDriver extends VcsDriver
             }
 
             $composer = $this->getBaseComposerInformation($identifier);
+            if ($composer) {
 
                 // specials for github
                 if (!isset($composer['support']['source'])) {
@@ -161,6 +162,7 @@ class GitHubDriver extends VcsDriver
                 if (!isset($composer['support']['issues']) && $this->hasIssues) {
                     $composer['support']['issues'] = sprintf('https://%s/%s/%s/issues', $this->originUrl, $this->owner, $this->repository);
                 }
+            }
 
             if ($this->shouldCache($identifier)) {
                 $this->cache->write($identifier, json_encode($composer));