Problem:
`mousekey_task` spams empty hid reports with when a mouse key is
pressed, causing resource exhaustion in the USB mouse endpoint.
Cause:
The check whether or not to send a new mouse report would always
evaluate to true if a mouse key is pressed:
1. `mouse_report` has non-zero fields and `tmpmr` is a copy of this
fields.
2. `mouse_report` is set to zero, `tmpmr` has now non-zero fields.
3. `has_mouse_report_changed` compares the two and evaluates to true
4. a mouse report is sent.
Fix:
The check condition of `has_mouse_report_changed` will evaluate any
empty record as unchanged, as mouse report data is relative and doesn't
need to return to zero. An empty report will still be send by
`register_mouse` on release of all mouse buttons.
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