Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

modules/graceful/net_unix.go

@@ -290,11 +290,11 @@ func getActiveListenersToUnlink() []bool {
 func getNotifySocket() (*net.UnixConn, error) {
 	if err := getProvidedFDs(); err != nil {
 		// This error will be logged elsewhere
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil when no provided FDs are available
 	}
 
 	if notifySocketAddr == "" {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil when notify socket is not configured
 	}
 
 	socketAddr := &net.UnixAddr{