Fixes #11545 - Fix the null safety issue in shimmer example in cookbook

[kishan-dhankecha]

Issues fixed by this PR (if any): #11545

I am not sure if I need to make changes in all the files that I changed or not so I'll keep this change in the Draft until someone corrects me If I made the wrong changes.

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[kishan-dhankecha]

Note that this issue of null check only happens when a user forgets to wrap the widget with Shimmer. In that case, context will not find the Shimmer on the upper side of the tree.