fix: avoid panic when no outputs are present

Replace `unwrap()` on the output scale query with conditional handling.
This prevents a panic when the last output is removed and the query
temporarily returns no results + add test to cover removing all outputs.

src/server/tests.rs

@@ -1773,6 +1773,17 @@ fn output_offset_remove_output() {
     check_output_position_event(&output_main_c, (0, 0));
 }
 
+#[test]
+fn remove_all_outputs() {
+    let (mut f, _) = TestFixture::new_with_compositor();
+
+    let (_, output) = f.new_output(0, 0);
+    f.run();
+
+    f.remove_output(output);
+    f.run();
+}
+
 #[test]
 fn output_offset_surface_positioning() {
     let (mut f, comp) = TestFixture::new_with_compositor();