Auto merge of #14341 - stshine:where-is-the-top, r=mrobinson,emilio

script: Fix the scroll to top behavior

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When finding the indicated fragment, do not use the document element to indicate
the top of the Document, and when scrolling to the frament and we do not find a
element, scrolling the top if the fragment is empty or equal to "top".

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because _____

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -14810,6 +14810,12 @@
             "url": "/_mozilla/mozilla/scrollTo.html"
           }
         ],
+        "mozilla/scroll_top_null_target.html": [
+          {
+            "path": "mozilla/scroll_top_null_target.html",
+            "url": "/_mozilla/mozilla/scroll_top_null_target.html"
+          }
+        ],
         "mozilla/send-arraybuffer.htm": [
           {
             "path": "mozilla/send-arraybuffer.htm",