libcamera: Use Size::isNull()

Use the new Size::isNull() function through the code base to replace
manual checks. While the new code isn't equivalent, as isNull() checks
that both width and height are zero, it catches the same conditions in
practice.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Umang Jain <email@uajain.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

test/v4l2_subdevice/test_formats.cpp

@@ -67,7 +67,7 @@ int FormatHandlingTest::run()
 		return TestFail;
 	}
 
-	if (format.size.width == 0 || format.size.height == 0) {
+	if (format.size.isNull()) {
 		cerr << "Failed to update image format" << endl;
 		return TestFail;
 	}