Fix: don't crash on missing wallet icons

[katspaugh]

What it solves

We see people using mystery wallets (the Detected Wallet in onboard?), and having the app crashed on accessing a missing icon.

How this PR fixes it

Don't display any icon if connected to a mystery wallet. Chances are, this wallet might just work.

[github-actions]

CLA Assistant Lite All Contributors have signed the CLA.

[github-actions]

ESLint Summary View Full Report

Annotations are provided inline on the Files Changed tab. You can also see all annotations that were generated on the annotations page.

Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	0	N/A

• Result: ✅ success
• Annotations: 0 total

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^{Report generated by eslint-plus-action}

[coveralls]

Pull Request Test Coverage Report for Build 1480972643

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.008%) to 31.798%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/components/AppLayout/Header/components/WalletIcon/index.tsx	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/components/AppLayout/Header/components/WalletIcon/index.tsx	1	0%

Totals
Change from base Build 1480795842:	-0.008%
Covered Lines:	2977
Relevant Lines:	8350

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💛 - Coveralls

[github-actions]

Deployment links

🟠 Safe Rinkeby	⚪ Safe Mainnet	🟣 Safe Polygon	🟡 Safe BSC	⚫ Safe Arbitrum	🟢 Safe xDai

[github-actions]

E2E Tests Failed
Check the results here: https://github.com/gnosis/safe-react-e2e-tests/actions/runs/1481000285

Failed tests:

• ❌ Safe Balances Safe Balances