Fix NullPointerException calling sendToJSPostSaveEvent

[zwarm]

Fixes #20665

This PR introduces a null check for the activity. This change is made to prevent potential NullPointerExceptions that could occur if the activity is not available at the time of call.

We were unable to recreate this crash. See discussion p1713455342282519-slack-C012H19SZQ8 for the reasoning behind the solution implemented.

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To Test:

• Install the JP app from this PR
• Login
• Navigate to Posts
• Edit a post or create a new post
• Add images
• Set a featured image
• Update or publish the post

• Verify the app doesn't crash

Regression Notes

1. Potential unintended areas of impact Edit/create post crashes

2. What I did to test those areas of impact (or what existing automated tests I relied on) Manual testing

3. What automated tests I added (or what prevented me from doing so) N/A

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PR Submission Checklist:

• [X] I have completed the Regression Notes.
• [X] I have considered adding accessibility improvements for my changes.
• [X] I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

---

Testing Checklist (strike-out the not-applying and unnecessary ones): N/A

[dangermattic]

	1 Warning
:warning:	This PR is assigned to the milestone 24.7 ❄️. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

Generated by :no_entry_sign: Danger

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr20677-d911084
	Commit	d9110848d567673babf282e545b95c9e5f294198
	Direct Download	jetpack-prototype-build-pr20677-d911084.apk

Note: Google Login is not supported on these builds.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr20677-d911084
	Commit	d9110848d567673babf282e545b95c9e5f294198
	Direct Download	wordpress-prototype-build-pr20677-d911084.apk

Note: Google Login is not supported on these builds.