fix: adding message to an empty message list

[myandrienko]

The refactor of the addToMessageList function made in #1330 contained an unfortunate error which prevents adding a message to an empty message list.

See:

• https://github.com/GetStream/stream-chat-js/pull/1330/files#diff-39b2554fd18da165b59a6351b1aafff3714e2a80c1435f2de9706355b4d32351L331-R388
• https://github.com/GetStream/stream-chat-js/pull/1330/files#diff-39b2554fd18da165b59a6351b1aafff3714e2a80c1435f2de9706355b4d32351R397-R399

[github-actions]

Size Change: +65 B (+0.02%)

Total Size: 426 kB

Filename	Size	Change
dist/browser.es.js	92.5 kB	+13 B (+0.01%)
dist/browser.full-bundle.min.js	54.1 kB	+9 B (+0.02%)
dist/browser.js	93.6 kB	+16 B (+0.02%)
dist/index.es.js	92.6 kB	+13 B (+0.01%)
dist/index.js	93.7 kB	+14 B (+0.01%)

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