roots_handle_shell_reveal: Null-check wlr_output_layout_output_at result
Every wlr_output is going to have an associated PhocOutput, but
wlr_output_layout_output_at is not guaranteed to always return
a non-NULL value. Therefore it doesn't make much sense to check
for output
being NULL right after unchecked dereference
of wlr_output
.