fix: DIA-2098: [mobx-state-tree] Map.put cannot be used to set empty state

[niklub]

Fixing sentry bug https://humansignal.sentry.io/issues/6164973783/?project=5848958&query=is%3Aunresolved%20issue.priority%3A%5Bhigh%2C%20medium%5D&referrer=issue-stream&sort=date&stream_index=5

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[hlomzik]

This fixes errors recorded in Sentry, but the original issue might still remain. Tests are using just a synthetic data, so also don't help much.
I'd say we need to investigate this deeper when we have time, but as a fast response it's a good one, approving!