vulkan: Remove unnecessary null checks

vulkan: Remove unnecessary null checks

These null checkes are slightly misleading when double-checking
mutability for external language interop.  None of the functions in
these files allow the variable at hand to become `NULL` under normal
operation, because they are checked at initialization and never (allowed
to be) reassigned to `NULL`.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1615>