Fix partial fromJust in votingScriptWitnessSingleton

[Jimbo4350]

Bug

votingScriptWitnessSingleton calls fromJust on the result of getVotingScriptCredentials, which returns Nothing when the VotingProcedures map is empty. Passing an empty VotingProcedures paired with a witness crashes at runtime.

The bug exists in two places:

• Cardano.Api.Experimental.Tx.Internal.BodyContent.New:529 — full bug: no Maybe guard on the witness at all
• Cardano.Api.Tx.Internal.Body:762 — partial fix: Nothing witness is handled, but Just witness paired with empty procedures still crashes

Fix

votingScriptWitnessSingleton should return Map.empty when getVotingScriptCredentials returns Nothing, rather than calling fromJust.