Auto merge of #18099 - KuoE0:make-empty-rule-can-be-inserted-into-rule-tree, r=emilio

Make emtpy rule can be inserted into rule tree

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We really want to ensure empty rule nodes appear in the rule tree for devtools, this condition ensures that if we find an empty rule node, we insert it at the normal level.

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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 [Bug 1384802](https://bugzilla.mozilla.org/show_bug.cgi?id=1384802)

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because of the test cases are in Gecko.

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