Masks with Z or T null should apply across all Z or T indices

[will-moore]

See #82 (comment)

Currently, if all masks have no Z or T index, the omero zarr masks Image:ID exports a label data block with size-Z or size-T = 1. However, this relies on all other clients to interpret this in the same way that OMERO does (to display that label image across all Z/T indices).
Instead, it it better to export a better representation of how we want the data to appear. If a Mask should apply across all Z or T planes, then we should export a Label that spans all those Z/T planes.

cc @pwalczysko

[sbesson]

A couple of additional notes from the discussion of this morning:

1. the OME model unambiguously specifies empty values of TheZ and/or TheT means the shape should apply to all Z-sections and/or time points

https://www.openmicroscopy.org/Schemas/Documentation/Generated/OME-2016-06/ome_xsd.html#Shape_TheZ
https://www.openmicroscopy.org/Schemas/Documentation/Generated/OME-2016-06/ome_xsd.html#Shape_TheT

2. I am unaware of any provision in the Zarr specification or the NGFF spec to handle these types of scenarios and re-use existing chunks for other indices