Forms: fixes php warning

[enejb]

Fixes a PHP warning where the that shows up when functions are expecting to be able to access $fields['_feedback_all_fields']

Proposed changes:

• Add the $fields['_feedback_all_fields'] always.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

Does this pull request change what data or activity we track or use?

no

Testing instructions:

Do the tests pass?

[github-actions]

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[github-actions]

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/forms/src/contact-form/class-contact-form-plugin.php	419/1043 (40.17%)	0.06%	0 💚

Full summary · PHP report · JS report