[V132-specific] Only spend UTXO unused gas for Migrate outbounds
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[V132-specific]
Intended to close #1878 (closed)
'Commit e8fe6be0 appears to avoidably send unused gas to UTXO outbound recipients'.
(This only affects the consolidated UTXO client code as the chain-specific client code has conveniently already been cleared in !3378 (merged).)
(Please note that the TxMigrate check doesn't need a simultaneous TxConsolidate check as that already exists just below;
it assumes MaxGas to be empty for TxConsolidate, unlike for TxMigrate.)