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

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