Fix `empty_row_vectorise` test on OpenCL

The code did not check to see if a vectorised matrix was empty before copying. This is the failing test (only failing for the OpenCL backend):

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main is a Catch v2.13.10 host application.
Run with -? for options

-------------------------------------------------------------------------------
empty_row_vectorise
-------------------------------------------------------------------------------
vectorise.cpp:98
...............................................................................

vectorise.cpp:98: FAILED:
due to unexpected exception with message:
  coot::opencl::copy_array(): couldn't copy buffer

I fixed it by adding size checks to copy_array() (and also copy_subview() while I was in there).