Auto merge of #21222 - nupurbaghel:update_source_set, r=jdm

Null image source check for src, srcset and picture parent

<!-- Please describe your changes on the following line: -->
This PR includes a test which checks for empty src only.
---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #21221

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- [x] These changes require tests and added

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tests/wpt/metadata/MANIFEST.json

@@ -357185,6 +357185,12 @@
      {}
     ]
    ],
+   "html/semantics/embedded-content/the-img-element/null-image-source.html": [
+    [
+     "/html/semantics/embedded-content/the-img-element/null-image-source.html",
+     {}
+    ]
+   ],
    "html/semantics/embedded-content/the-img-element/relevant-mutations.html": [
     [
      "/html/semantics/embedded-content/the-img-element/relevant-mutations.html",
@@ -597948,6 +597954,10 @@
    "4d929fd8b160205f1f0fd7ab019cbb31411c69ad",
    "testharness"
   ],
+  "html/semantics/embedded-content/the-img-element/null-image-source.html": [
+   "89992765036636c2b9eafbe9b579214fcc55346f",
+   "testharness"
+  ],
   "html/semantics/embedded-content/the-img-element/relevant-mutations.html": [
    "c8e09e8d0b8a7abbec87632b9f55240505d1ccc7",
    "testharness"