fix(keymap): Fix null dereference with debug logging

[renshyle]

If a layer contains fewer sensor bindings than there are sensors declared, binding->behavior_dev is null when a sensor without a behavior in an active layer's bindings is triggered. Fix this by logging a placeholder name if the binding does not exist.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.

[petejohanson]

Thanks!