Empty subset result should maintain dimensions

[skorper]

When a subset is requested that is outside of the bounds of the input granule, an empty granule is returned. This empty granule is a shell of the input granule and contains all of the same variable attributes, global attributes, etc., but has empty data.

One issue we are seeing however is that the empty granule doesn't maintain its dimensions. Those dimensions should be maintained which will allow the granule to be used with Harmony concatation.

For example:

➜ ncdump -h ascat_20211220_231800_metopb_48038_eps_o_250_3202_ovw.l2.nc4
netcdf ascat_20211220_231800_metopb_48038_eps_o_250_3202_ovw.l2 {
dimensions:
	time = UNLIMITED ; // (0 currently)
	lat = UNLIMITED ; // (0 currently)
	lon = UNLIMITED ; // (0 currently)
	wind_speed = UNLIMITED ; // (0 currently)
	wind_dir = UNLIMITED ; // (0 currently)
variables:
	double time(time) ;
		time:_FillValue = 1.00000001504747e+30 ;
		time:missing_value = 1.e+30 ;
		time:valid_min = 0 ;
		time:valid_max = 2147483647 ;
		time:standard_name = "time" ;
		time:long_name = "time" ;
		time:coordinates = "lat lon" ;
	double lat(lat) ;
		lat:_FillValue = 1.00000001504747e+30 ;
		lat:missing_value = 1.e+30 ;
		lat:valid_min = -9000000 ;
		lat:valid_max = 9000000 ;
		lat:standard_name = "latitude" ;
		lat:long_name = "latitude" ;
		lat:units = "degrees_north" ;
	double lon(lon) ;
		lon:_FillValue = 1.00000001504747e+30 ;
		lon:missing_value = 1.e+30 ;
		lon:valid_min = 0 ;
		lon:valid_max = 36000000 ;
		lon:standard_name = "longitude" ;
		lon:long_name = "longitude" ;
		lon:units = "degrees_east" ;
	double wind_speed(wind_speed) ;
		wind_speed:_FillValue = 1.00000001504747e+30 ;
		wind_speed:missing_value = 1.e+30 ;
		wind_speed:valid_min = 0s ;
		wind_speed:valid_max = 5000s ;
		wind_speed:standard_name = "wind_speed" ;
		wind_speed:long_name = "wind speed at 10 m" ;
		wind_speed:units = "m s-1" ;
		wind_speed:coordinates = "lat lon" ;
...