Fix None values getting exported as empty strings

[noissue]

pulp_deb/app/modelresource.py

@@ -3,6 +3,7 @@
 
 from pulpcore.plugin.importexport import BaseContentResource
 from pulpcore.plugin.modelresources import RepositoryResource
+from pulp_deb.app.constants import NULL_VALUE
 from pulp_deb.app.models import (
     AptRepository,
     GenericContent,
@@ -53,6 +54,38 @@ def set_up_queryset(self):
 
         return content
 
+    def dehydrate_field(self, obj, field_name):
+        """
+        Assigns fields that don't have a value (e.g. None) with the special `NULL_VALUE`.
+        
+        This ensures that the import can set it correctly to None rather than an empty string.
+
+        Args:
+            obj: exporting object.
+            field_name (str): name of the field that gets exported.
+
+        Returns:
+            str: the field value or NULL_VALUE.
+        """
+        field = self.fields[field_name]
+        value = field.attribute and field.get_value(obj) or None
+        return value if value is not None else NULL_VALUE
+
+
+    def before_import_row(self, row, **kwargs):
+        """
+        Ensures that all fields that got exported with the `NULL_VALUE` will be imported as None.
+
+        Args:
+            row (tablib.Dataset row): incoming import-row representing a single PRC.
+            kwargs: args passed along from the import() call.
+        """
+        super().before_import_row(row, **kwargs)
+
+        for key, value in row.items():
+            if value == NULL_VALUE:
+                row[key] = None
+
 
 class InstallerFileIndexResource(DebContentResource):
     """