[clang] [llvm] [VFS] Guard against null key/value nodes when parsing YAML overlay (PR #190506)

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github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Windows-->
# :window: Windows x64 Test Results

* 133063 tests passed
* 3061 tests skipped
* 1 test failed

## Failed Tests
(click on a test name to see its output)

### LLVM-Unit
<details>
<summary>LLVM-Unit.Support/_/SupportTests_exe/VFSFromYAMLTest/BasicVFSFromYAML</summary>

```
Script:
--
C:\_work\llvm-project\llvm-project\build\unittests\Support\.\SupportTests.exe --gtest_filter=VFSFromYAMLTest.BasicVFSFromYAML
--
C:\_work\llvm-project\llvm-project\llvm\unittests\Support\VirtualFileSystemTest.cpp:1596
Expected equality of these values:
  3
  NumDiagnostics
    Which is: 4


```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/190506