feat: add an EMPTY ecosystem for withdrawn records (#2705)

Some records will be withdrawn from upstream and may also have their
affected package information deleted. These records currently cannot be
exported and may cause confusion for our customers due to missing
records. Adding an "EMPTY" ecosystem will represent these records for
exporting.

osv/models.py

@@ -396,6 +396,11 @@ def _pre_put_hook(self):  # pylint: disable=arguments-differ
         if 'GIT' in ecosystems_set:
           break
 
+    # If a withdrawn record has no affected package,
+    # assign an '[EMPTY]' ecosystem value for export.
+    if not ecosystems_set:
+      ecosystems_set.add('[EMPTY]')
+
     # For all ecosystems that specify a specific version with colon,
     # also add the base name
     ecosystems_set.update({ecosystems.normalize(x) for x in ecosystems_set})