Fix AccessibilityNode.hasOnlyUnloadedChildren()
Fix AccessibilityNode.hasOnlyUnloadedChildren()

axNodeForId used to return undefined for IDs it didn't know about, but
now it returns null. Fix hasOnlyUnloadedChildren to check for null
instead of undefined.

Change-Id: I2552ebffc8ba6c857199e281aaa8a4d882a5be63
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2505344
Reviewed-by: Peter Marshall <6d0eecbeb7fb9d172f49b4b32481d2466fd89429@chromium.org>
Reviewed-by: Changhao Han <68542faf32f0a558a873c0f094ca9bedb2744bff@chromium.org>
Commit-Queue: Changhao Han <68542faf32f0a558a873c0f094ca9bedb2744bff@chromium.org>

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