fix(packagist): ignore null versions

lib/datasource/packagist.js

@@ -81,6 +81,10 @@ async function getPackagistFile(regUrl, file) {
 
 function extractDepReleases(versions) {
   const dep = {};
+  // istanbul ignore if
+  if (!versions) {
+    return dep;
+  }
   dep.releases = Object.keys(versions)
     .filter(isVersion)
     .sort(sortVersions)