[CHIA-1219] Allow coin selection of 0 value coins

[Quexington]

There was a bad heuristic in the coin selection algorithm to check for a lack of coins. This fixes that and adds a test to make sure that 0 value coins can be selected.

[coveralls-official]

Pull Request Test Coverage Report for Build 12895448216

Details

• 6 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• 37 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.001%) to 91.513%

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Files with Coverage Reduction	New Missed Lines	%
chia/plotters/plotters.py	1	90.94%
chia/full_node/full_node_api.py	1	84.38%
chia/wallet/wallet_node.py	1	88.25%
chia/consensus/blockchain.py	2	94.14%
chia/timelord/timelord_launcher.py	2	89.29%
chia/full_node/full_node.py	4	86.22%
chia/server/node_discovery.py	6	80.77%
chia/plotters/madmax.py	6	44.58%
chia/timelord/timelord.py	14	78.2%

Totals
Change from base Build 12894248841:	-0.001%
Covered Lines:	105324
Relevant Lines:	114913

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💛 - Coveralls