Fix server crash when no local plugins provided

Bug: Issue 6579
Change-Id: I9e90d21f868967097a533cefd6dba4b5260b6956
diff --git a/polygerrit-ui/server.go b/polygerrit-ui/server.go
index f2a97bf..a42acc3 100644
--- a/polygerrit-ui/server.go
+++ b/polygerrit-ui/server.go
@@ -122,6 +122,9 @@
 }
 
 func injectLocalPlugins(r io.Reader) io.Reader {
+	if len(*plugins) == 0 {
+		return r
+	}
 	// Skip escape prefix
 	io.CopyN(ioutil.Discard, r, 5)
 	dec := json.NewDecoder(r)