Fix undefined checking for stripe store in Stripe Checkout Block

[Soare-Robert-Daniel]

Closes #1815

Summary

Added ?. as a guard against undefined values when functions are not yet initialized.

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Test instructions

ℹ️ You need to test on WP5.9. Make sure to have a Stripe API Key for testing.

1. Insert a Stripe Checkout Block
2. It should work as expected.

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Checklist before the final review

• Included E2E or unit tests for the changes in this PR.
• Visual elements are not affected by independent changes.
• It is at least compatible with the minimum WordPress version.
• It loads additional script in frontend only if it is required.
• Does not impact the Core Web Vitals.
• In case of deprecation, old blocks are safely migrated.
• It is usable in Widgets and FSE.
• Copy/Paste is working if the attributes are modified.
• PR is following the best practices

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Bundle Size Diff

Package	Old Size	New Size	Diff
Animations	270.78 KB	270.78 KB	0 B (0.00%)
Blocks	1.41 MB	1.41 MB	14 B (0.00%)
CSS	85.28 KB	85.28 KB	0 B (0.00%)
Dashboard	122.49 KB	122.49 KB	0 B (0.00%)
Export Import	83.17 KB	83.17 KB	0 B (0.00%)
Pro	302.52 KB	302.52 KB	0 B (0.00%)

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E2E Summary

Typing

Test	Average Time (ms)	Standard Deviation (ms)	Median Time (ms)	Quantile for soft limit (%)	Quantile for hard limit (%)
Typing	76.56	6.13	76.08	0 (60ms)	73.68 (80ms)

Values above 60ms

"0 - 79.92, 1 - 71.48, 2 - 76.08, 3 - 81.68, 4 - 74.76, 5 - 66.08, 6 - 85.75, 7 - 76.59, 8 - 72.76, 9 - 86.12, 10 - 78.99, 11 - 79.56, 12 - 71.78, 13 - 81.06, 14 - 69.01, 15 - 69.81, 16 - 88.13, 17 - 69.45, 18 - 75.56"

[irinelenache]

@Soare-Robert-Daniel Tested and the issue is fixed now, thank you 🚀

[pirate-bot]

🎉 This PR is included in version 2.3.4 🎉

The release is available on GitHub release

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