[#257] Prevent empty transactions in transfer proposals

[rinn7e]

Description

Problem: Both treasuryDAO and registryDAO have proposals that can support xtz transfers. Both implement handle_transfer for Xtz_transfer_type in a way that can fail: empty transactions (of unit parameter an 0 amount) will be rejected by the network.

Solution: proposal_check now reject proposal that contains Xtz_transfer_type with 0 mutez, add tests to cover this.

Related issue(s)

Resolves #257

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