Partially addresses #6726

• i.e. the merge bit.

This is thought to be needed for #6727

• since that defines loading dataless cubes from netcdf.
• but that can only work is we can at least pass-through dataless cubes in merging (as it's implied in loading).

I think this now covers all that is asked for regarding merge in #6726

• dataless cubes can pass through merge
• multiple dataless cubes can be merged to a dataless result
• dataless can combine with 'dataful' cubes in merging, producing masked points in that region of cube.data