ExternalCameraHAL: Check for empty native handle instead of null.

AIDL cannot send nullptr when parceling NativeHandles. Instead, it
sends an empty  handle. Logic which previously relied on null
checks should now rely on checking if the native_handle is
empty or not.

This CL updates ExternalCameraUtils to check for empty handles
instead.

Bug: 299170474
Test: CameraAidlTest#processCaptureRequestInvalidBuffer passes
Change-Id: Id4ba928defaa061708a3c1cd68aa510b376f6cb3