Actions without keys should be rejected

[lhecker]

Extracted from #10933.

Steps to reproduce

Add this to your "actions":

{
    "command": "unbound",
    "keys": ""
}

Expected behavior

The application should ignore/reject such actions and optionally remove or warn about them.

Actual behavior

ActionMap is being filled with such actions without validation.
Additionally the bug reporter (@Rosefield) mentioned that they can't remember adding such an action by themselves. It might thus be necessary to add further assert()s to find further causes elsewhere.