Use XLogRecPtrIsValid() consistently for WAL position checks

Use XLogRecPtrIsValid() consistently for WAL position checks

Commit a2b02293bc6 switched various checks to use XLogRecPtrIsValid(),
but later changes reintroduced XLogRecPtrIsInvalid() and direct comparisons
with InvalidXLogRecPtr.

This commit replaces those uses with XLogRecPtrIsValid() for better
readability and consistency.

Author: Vignesh C <vignesh21@gmail.com>
Reviewed-by: Xiaopeng Wang <wxp_728@163.com>
Reviewed-by: Amul Sul <sulamul@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CALDaNm16knMFtcqyAG3XYSkyagmVXfhaR0T=hau8UTAU0+eLQQ@mail.gmail.com