[SYCL] Initialize uninitialized handler_impl fields (#15323)
In the same vein as https://github.com/intel/llvm/pull/15237, this PR
fixes additional uninitialized values recently discovered by Coverity.
Similar to the resolution discussed in #15237, I have
default-initialized integer values that are defined later on to 0
instead of another more complex solution.
Additionally, since I had set `MExternalSempahore` in `handler_impl` to
`nullptr`, I added null checks where `MExternalSemaphore` is ultimately
returned to ensure `nullptr` doesn't actually get passed into the UR.
This is not necessarily necessary, but without this check Coverity would
probably generate another hit because of it.
