Actions without keys should be rejected

Extracted from #10933.

# Steps to reproduce

Add this to your "actions":
```json
{
"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.

Start by tracing where ActionMap is filled and where actions from the configuration are validated. Reproduce the empty-keys action, then verify that it is rejected or ignored and that any resulting warning or assertion behavior is covered by the relevant existing tests.

Written by the indexing model from the issue text.