fix: Add optional chaining to currencyRates check for stability

[gambinish]

Description

#28750

Related issues

Fixes: #28750

Manual testing steps

Testing steps in issue. Behavior is inconsistent, hard to replicate consistently.

Screenshots/Recordings

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Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
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[github-actions]

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[metamaskbot]

Builds ready [24c2ecd]

• builds: chrome, firefox
• builds (beta): chrome
• builds (flask): chrome, firefox
• builds (MMI): chrome, firefox
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Background Module Init Stats
• mv3: UI Init Stats
• mv3: Module Load Stats
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• code coverage: Report
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7
  • common: 0, 1, 10, 11, 12, 2, 3, 4, 5, 6, 7, 8, 9
  • content-script: 0
  • offscreen: 0
  • ui: 0, 1, 10, 11, 12, 2, 3, 4, 5, 6, 7, 8, 9

Page Load Metrics (1725 ± 98 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	320	2154	1662	364	175
		domContentLoaded	1471	2100	1689	186	89
		load	1498	2156	1725	205	98
		domInteractive	21	83	41	18	9
		backgroundConnect	5	108	36	28	13
		firstReactRender	16	74	36	23	11
		getState	5	66	21	19	9
		initialActions	0	0	0	0	0
		loadScripts	1048	1583	1254	158	76
		setupStore	6	57	14	16	8
		uiStartup	1679	2349	1918	200	96

Bundle size diffs

• background: 0 Bytes (0.00%)
• ui: 81 Bytes (0.00%)
• common: 0 Bytes (0.00%)

[sahar-fehri]

Were you able to repro the linked issue locally?
From the error linked in the issue i think this optional chaining makes sense

[gambinish]

I wasn't able to reproduce exactly, but I've definitely seen similar bugs from time to time related to currencyRates not yet resolving.

[salimtb]

LGTM ✅ I couldn't reproduce the issue, but the safe accessor will help us prevent this error.