Field/domain print fails when dimension corodinate has no data

[davidhassell]

(First reported over at NCAS-CMS/cf-python#326 - see there for full details)

When a field has a construct which can have data, e.g. a coordinate, but that construct doesn't have data, the str() representation hits a ValueError regarding the lack of data on that construct, e.g:

>>> import cfdm
>>> f = cfdm.example_field(2)
>>> t = f.construct('time')
>>> t.del_data()
<Data(36): [1959-12-16 12:00:00, ..., 1962-11-16 00:00:00]>
>>> print(f)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-fc0364975534> in <module>
----> 1 print(f)

~/miniconda3.9/lib/python3.9/site-packages/cfdm/field.py in __str__(self)
    239             string.append(f"Field ancils    : {field_ancils}")
    240 
--> 241         string.append(str(self.domain))
    242 
    243         return "\n".join(string)

~/miniconda3.9/lib/python3.9/site-packages/cfdm/domain.py in __str__(self)
    194                 if construct_data_axes[cid] == (axis_cid,):
    195                     name = dim.identity(default=f"key%{0}")
--> 196                     y = f"{name}({dim.get_data().size})"
    197                     if y != axis_names[axis_cid]:
    198                         y = f"{name}({axis_names[axis_cid]})"

~/miniconda3.9/lib/python3.9/site-packages/cfdm/core/abstract/propertiesdata.py in get_data(self, default, _units, _fill_value)
    290                 return
    291 
--> 292             return self._default(
    293                 default, message=f"{self.__class__.__name__} has no data"
    294             )

~/miniconda3.9/lib/python3.9/site-packages/cfdm/core/abstract/container.py in _default(self, default, message)
    145                 default = type(default)(message)
    146 
--> 147             raise default
    148 
    149         return default

ValueError: DimensionCoordinate has no data

>>>

Something sensible should be displayed, instead.

[davidhassell]

Of the possible solutions described at NCAS-CMS/cf-python#326, I favour:

>>> print(f)
Field: air_potential_temperature (ncvar%air_potential_temperature)
------------------------------------------------------------------
Data            : air_potential_temperature(time(36), latitude(5), longitude(8)) K
Cell methods    : area: mean
Dimension coords: time(36) = 
                : latitude(5) = [-75.0, ..., 75.0] degrees_north
                : longitude(8) = [22.5, ..., 337.5] degrees_east
                : air_pressure(1) = [850.0] hPa

as it is the one which identifies the construct as "time", but doesn't introduce any other notions.