Add plugin tests for mjSpec.

Fixes #1903.

PiperOrigin-RevId: 663352920
Change-Id: I3a9e17aec5694e49c20b542f4b0a4402f88a2f17

src/user/user_api.cc

@@ -606,6 +606,9 @@ const char* mjs_resolveOrientation(double quat[4], mjtByte degree, const char* s
 
 // get id
 int mjs_getId(mjsElement* element) {
+  if (!element) {
+    return -1;
+  }
   return static_cast<mjCBase*>(element)->id;
 }