Auto merge of #26943 - jdm:promise-incumbent, r=Manishearth

Fix panic on vimeo.com

The only time we end up enqueuing a promise reaction job with a null incumbent global argument is when SpiderMonkey does some weird stuff behind the scenes for its JS Debugger support (based on https://searchfox.org/mozilla-central/rev/2c1092dc68c63f7bad6da6a03c5883a5ab5ff2ca/js/src/builtin/Promise.cpp#3168). This commit works around that case and adds a regression test so we don't forget about that in the future.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #26930
- [x] There are tests for these changes

tests/wpt/mozilla/meta/MANIFEST.json

@@ -14222,6 +14222,13 @@
       {}
      ]
     ],
+    "promise-no-incumbent.html": [
+     "a5458d096b5da8d6619ef5c553f232261887f45a",
+     [
+      null,
+      {}
+     ]
+    ],
     "promise.html": [
      "5925047287175f291c07ee88075359ed92a86e6a",
      [