slight simplification to get_min_fee_rate()

[arvidn]

Purpose:

instead of throwing an exception from get_min_fee_rate(), return None.

Current Behavior:

get_min_fee_rate() may throw an exception if the mempool limit is set small.

New Behavior:

get_min_fee_rate() does not throw an exception because the mempool limit is set small.

[github-actions]

File	Coverage	Missing Lines
chia/full_node/mempool_manager.py	88.9%	lines 471

Total	Missing	Coverage
22 lines	Unknown	95%