Change get_data_in_grid return value when no data in grid

[jflowernet]

At the moment get_data_in_grid errors with a message when a spatial grid has no data within it. This creates problems when looping over data to be gridded as it stops the loop. For sf grids, @jase would like the spatial grid that was input returned unchanged, with just a message saying there is no data.
For raster grids, I think it should either return an NA raster or NULL - perhaps the former since if stacking results of many calls to get_data_in_grid, could then filter out NAs with stack[[!is.na(terra::global(stack, min)[[1]])]]

Will fix emlab-ucsb/oceandatr#41