[#746] Fail emulator on transfers from empty sender

NB: I've opted to make the change in Morley's runtime directly. I'm not aware of any workflows that might break, but I don't claim to know everything. If this might be problematic, we can move the check to Cleveland's side of things, although that would be somewhat more convoluted IMO.

Description

Problem: The real network doesn't allow transfers from implicit addresses with 0 balance. We did explicitly allow this on emulator in issue #15 (closed), however our testing framework changed a lot since then, and this difference in behavior is undesirable.

Solution: Check if a key address is unknown or has no funds on transfer; fail with EEEmptyImplicitContract.

Related issue(s)

Resolves #746 (closed)

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