Baker: fix min delta at end of round

What

This MR fix the computation of a delta time to wait at the end of a round before injecting next round block.

Why

If the minimal_block_delay < 5 then the delta is 0 and the bakers do not wait before injecting next round block.

How

By adding a minimal of 1 to the delta

Manually testing the MR

CI green.

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).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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