[net7.0] Null check everything in InvalidateMeasureIfContentSizeChanged

[github-actions]

Backport of #17472 to net7.0

/cc @PureWeen @hartez

[PureWeen]

The original PR expressed some concern around this not being an actual fix #17472

While there is some truth to this, this PR just adds some guards to extra code that was added here which would cause an NRE.

ViewWillLayoutSubViews was always getting called when the CollectionView was removed from the Window but the code added from the above PR now called some additional code that needs to be null checked. 14951 has been part of .NET 8 since preview 5; this is the only regression we've so far identified from that PR so we feel like it's a better story to keep that PR as part of SR8 and merge this fix in to manage that regression.

The issue has been narrowed down to a very specific piece of code here. If you comment out this code, then it no longer calls ViewWillLayoutSubviews

This path also only occurs on Shell when you set any styling elements.

I have the recipes app added into the main_recipes branch if you want to see this crash happen against our repository.

[PureWeen]

/azp run MAUI-DeviceTests-public

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