Fix CTS extension errors

- Add null check for capture and repeating surface targets before checking whether they are contained in the request target to prevent incorrect validation of the capture request targets
- Prevent checking validity of the postview surface if one is not requested to fix incorrect 'postview feature is not available' error

Test: CTS test
Bug: 264321351
Change-Id: Ia8a4a2d12d3ec6748c3bf2004159306127985dd8