Fix crash in PostSearchViewModel

[kean]

Fixes https://a8c.sentry.io/issues/1871799689/events/caa357e5119945fbbd9967901d2d899d/

To test:

• Test that posts still show up in search results

Regression Notes

1. Potential unintended areas of impact

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

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

PR submission checklist:

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

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[kean]

The existing defensive code was not enough to protected from the scenario when there are duplicated within a single response (posts). This is what I assume is causing the crash.

[wpmobilebot]

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

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23247-be5aa1b
	Version	24.9
	Bundle ID	org.wordpress.alpha
	Commit	be5aa1b
	App Center Build	WPiOS - One-Offs #9974

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[wpmobilebot]

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

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23247-be5aa1b
	Version	24.9
	Bundle ID	com.jetpack.alpha
	Commit	be5aa1b
	App Center Build	jetpack-installable-builds #9024

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