Proto: Refactor empty transactions

Context

This Mr addresses issues #639 and #645.

The detection of empty transactions is moved out of contract_storage to avoid having to test that an amount of tez that must be transferred is different from zero in situations where that is clearly irrelevant (e.g. when the transfer is not resulting from a transaction operation).

We also take this opportunity to treat empty transfers as no-ops, which avoids the error described in #645.

Manually testing the MR

• CI, and
• the first commit has been back-ported to granada (see second commit here), and replayed on 20000 blocks starting from level 1794462. The change was exercised in the error case at least once (at level 1804665).