[CHIA-365] Return exception and error from get_kv_diff when either of the hashes has no data

[emlowe]

Fix issue where get_kv_diff returns an empty diff even though one of the provided hashes has no data. Instead this will return an error code about unable to perform the diff because there wasn't any data to diff.

[coveralls-official]

Pull Request Test Coverage Report for Build 9134173387

Details

• 15 of 15 (100.0%) changed or added relevant lines in 3 files are covered.
• 7 unchanged lines in 5 files lost coverage.
• Overall coverage increased (+0.04%) to 90.849%

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Files with Coverage Reduction	New Missed Lines	%
chia/daemon/keychain_proxy.py	1	72.57%
chia/farmer/farmer.py	1	72.23%
chia/wallet/wallet_node.py	1	88.72%
chia/data_layer/data_layer.py	2	85.32%
chia/daemon/client.py	2	73.33%

Totals
Change from base Build 9129839013:	0.04%
Covered Lines:	99217
Relevant Lines:	109152

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💛 - Coveralls

[arvidn]

looks good!