Doing null check before generating Font Size tooltip

Since the lifecycle of controller is independent of that of preference,
we do null check on the seekbar to avoid generating tooltip from the
seekbar when it is not ready. The fix is a temporary solution for the
issue and we would like to move the logic of tooltip to systemUI to
match the general view's lifecycle in the future.

Bug: 279549685
Test: manually - check the tooltip won't be shown and also won't cause exception after rotating the phone. (The seekbar view has not been created if it is out of screen.)
Test: manually - attach video to the bug
Test: make RunSettingsRoboTests ROBOTEST_FILTER=PreviewSizeSeekBarControllerTest
Change-Id: I50111f0cc59cad4401ab67b06cd62e9067e8ed76