MDL-46284 tool_messageinbound: Handle checkboxes correctly
MDL-46284 tool_messageinbound: Handle checkboxes correctly

When a checkbox is not checked, it does not submit in the form, therefore
this must be an !empty() test instead of an (int) casting.

This issue is a part of the MDL-47194 Task.
This issue is a part of the MDL-39707 Epic.

dilawar/moodle