fix(nuget): handle null versions

lib/datasource/nuget.js

@@ -19,7 +19,7 @@ async function getPkgReleases(purl) {
     const dep = {
       name,
     };
-    dep.releases = res.versions
+    dep.releases = (res.versions || [])
       .filter(isVersion)
       .sort(sortVersions)
       .map(version => ({ version }));