SMTChecker: Handle `call` with empty data in the same way as `send`

In particular, we should create a verification target for checking sufficient balance is available.

While `send` and `transfer` were not treated as untrusted external calls, I think they actually should have been treated as such (because of possible reentrancy).

The issue does not name files or tests. Start by locating SMTChecker handling for call, send, and transfer, then compare how empty-data calls are modeled; done means call with empty data creates a sufficient-balance verification target and send/transfer are treated as potentially reentrant external calls.

Written by the indexing model from the issue text.