Auto merge of #16063 - methyl:image-usemap-panics, r=nox

Avoid panics for empty or multibyte image usemap

<!-- Please describe your changes on the following line: -->
Some check were added to make sure we can call `split_at` with no risk of panics (when value is empty or the first char is multibyte).

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15883 (github issue number if applicable).
- [x] There are tests for these changes

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -12952,6 +12952,12 @@
      {}
     ]
    ],
+   "mozilla/htmlimageelement.html": [
+    [
+     "/_mozilla/mozilla/htmlimageelement.html",
+     {}
+    ]
+   ],
    "mozilla/htmllabel-activation.html": [
     [
      "/_mozilla/mozilla/htmllabel-activation.html",
@@ -25499,6 +25505,10 @@
    "54fe6bbc1a7a35ceb14a3bf33f81ebed55beaf72",
    "testharness"
   ],
+  "mozilla/htmlimageelement.html": [
+   "f1857a8f413f444b34464f0a3bc6b46af13c1a5a",
+   "testharness"
+  ],
   "mozilla/htmllabel-activation.html": [
    "2013b49684d0b861415a2e766d5d260906b1f3fb",
    "testharness"