getGridSize() handles plates with no Wells

[will-moore]

Fixes error from QA: https://www.openmicroscopy.org/qa2/qa/feedback/41352/

{"message": "'NoneType' object has no attribute 'val'", "stacktrace": "Traceback (most recent call last):
File \"/opt/omero/web/venv3/lib64/python3.9/site-packages/omeroweb/webgateway/views.py\", line 1500, in wrap
 rv = f(request, *args, **kwargs)
 File \"/opt/omero/web/venv3/lib64/python3.9/site-packages/omeroweb/webgateway/views.py\", line 1708, in plateGrid_json
 rv = plateGrid.metadata
 File \"/opt/omero/web/venv3/lib64/python3.9/site-packages/omeroweb/webgateway/plategrid.py\", line 71, in metadata
self.plate.setGridSizeConstraints(8, 12)
 File \"/opt/omero/web/venv3/lib64/python3.9/site-packages/omero/gateway/__init__.py\", line 6637, in setGridSizeConstraints
 gs = self.getGridSize()\n File \"/opt/omero/web/venv3/lib64/python3.9/site-packages/omero/gateway/__init__.py\", line 6659, in getGridSize
 self._gridSize = {'rows': row.val+1, 'columns': col.val+1}
AttributeError: 'NoneType' object has no attribute 'val'
"}

To test, create a Plate with no Wells:

$ omero obj new Plate name=empty

Browse in webclient:

[sbesson]

The handling makes sense. An alternative would be to store and return self._gridSize = None. In case the wells are being added in between calls, this would allow subsequent calls to self.getGridSize() to retry the plate query. I do not know enough about consumers of this API and how a None return value would be handled.

Either way, it's probably good practice to update the docstring and document the expectation in case the plate has no wells.

[will-moore]

@sbesson - apologies for the delay...
I think the expectation is not to return None - Returning sizes of 0 is more consistent and doesn't need special handling of None by consumers.
Docstring updated.