tests: check all combinations of internal, external and bounced receivers for "empty" messages

[jeshecdom]

Issue

Closes #1849.
Closes #1850.
Closes #1924.
Closes #1918.

For issue #1850, this PR tests it implicitly, because this PR checks all possible combinations of receivers (internal, external and bounced).

This PR sends "empty" messages to receivers:

• A message with an op code less than 32 bits.
• An actual empty cell (not even an opcode).
• A message with opcode 0, but with exactly 32 bits in length.
• The empty string.
• An empty string inside a Message struct.
• An empty message struct.

In the above I said "towards", because this PR is still not checking all possible combinations of bounced receivers.
Bounced receivers are now checked.

Checklist

• I have updated CHANGELOG.md
• I have run all the tests locally and no test failure was reported
• I have run the linter, formatter and spellchecker
• I did not do unrelated and/or undiscussed refactorings

[anton-trunov]

let's fix the merge conflicts here and try and merge it

[novusnota]

Fixed snapshots (hopefully)

[novusnota]

AWESOME STUFF @jeshecdom @anton-trunov

Triple