Avoid assuming that access_url exists always

DatalinkResultsMixin was assuming that the records created by any
subclass would have the `access_url` attribute. This is not true for SSA
(whether it should do is a separate question). This replaces that
assumption with a check for existence first, and then if that fails,
fall back to getdatalink on the record.

pyvo/dal/adhoc.py

@@ -218,10 +218,14 @@ def iter_datalinks(self):
                 processed_ids.append(id1)
                 remaining_ids.remove(id1)
                 yield current_batch.clone_byid(id1)
-            elif row.access_format == DATALINK_MIME_TYPE:
+            elif getattr(row, 'access_format', None) == DATALINK_MIME_TYPE:
                 yield DatalinkResults.from_result_url(row.getdataurl())
             else:
-                yield None
+                # Fall back to row-specific handling
+                try:
+                    yield row.getdatalink()
+                except AttributeError:
+                    yield None
 
 
 class DatalinkRecordMixin: