LayerMapping doesn't handle missing times, just missing date and datetime

[Jacob Walls]

When parsing GDAL LayerMapping objects, Field.as_datetime() returns None for an unset field. Both OFTDate.value and OFTDateTime.value catch the resulting unpacking TypeError, but OFTTime.value does not:

For example, adjust this recently merged test data edit for #36626 (this might actually be sufficient for a regression test, since this is all I wanted to do at the time of merging):

tests/gis_tests/data/has_nulls/has_nulls.geojson

@@ -59 +59 @@
       "type": "Feature",
       "properties": {
         "uuid": "fa2ba67c-a135-4338-b924-a9622b5d869f",
-        "time": "00:00:00",
         "integer": null,
         "num": null
       },

tests/gis_tests/gdal_tests/test_ds.py

@@ -110 +110 @@
                 None,
                 datetime.strptime("2018-11-29T03:02:52", datetime_format),
             ],
-            "time": [time(11, 32, 14, 123000), time(0), time(3, 2, 52)],
+            "time": [time(11, 32, 14, 123000), None, time(3, 2, 52)],
         },
         fids=range(3),
     ),

  File "/django/source/tests/gis_tests/gdal_tests/test_ds.py", line 199, in test03a_layers
    self.assertEqual(fld_value, layer.get_fields(fld_name))
    
  File "/django/source/django/contrib/gis/gdal/layer.py", line 212, in get_fields
    return [feat.get(field_name) for feat in self]
    ^^^^^^^^^^^^^^^
  File "/django/source/django/contrib/gis/gdal/feature.py", line 113, in get
    return self[field_name].value
    ^^^^^^^^^^^^^^^
  File "/django/source/django/contrib/gis/gdal/field.py", line 216, in value
    yy, mm, dd, hh, mn, ss, tz = self.as_datetime()
    ^^^^^^^^^^^
TypeError: cannot unpack non-iterable NoneType object

Seems like a minor oversight in 75d627888bf42f8de6064a0bd665c98c0df66c55 (#30020).

[Jacob Walls]

​PR