Fix bake for flakiness

Context

This MR makes the baker waits for the mempool to be retrieved before attempting to bake a block via bake for in order to avoid a race condition where an empty block would be produced.

Fixes #3097 (closed).

Alternative to !5341 (closed) and !5339 (closed)

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
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