[FIX] web: clear m2o input at creation cancel

After entering a value in a many2one, if one clicks somewhere else, a popup
is opened to suggest the user to create (or not) a new record with the entered
value.

Before this rev, closing this popup resulted in an unclear situation where the
input was still set with the entered value but the new record hadn't been created.

This commit fixes this by clearing the input value if the record is not created.

See task#36055