check prereg_links exists before raising exception (for current workflow it looks to be impossible 'why_no_prereg' be empty and 'prereg_links' exists simultaniously) (#11492)

Author: mkovalua

api/preprints/serializers.py

@@ -325,6 +325,7 @@ def update(self, preprint, validated_data):
 
         updated_has_prereg_links = validated_data.get('has_prereg_links', preprint.has_prereg_links)
         updated_why_no_prereg = validated_data.get('why_no_prereg', preprint.why_no_prereg)
+        prereg_links = validated_data.get('prereg_links', preprint.prereg_links)
 
         if updated_has_coi is False and updated_conflict_statement:
             raise exceptions.ValidationError(
@@ -341,7 +342,7 @@ def update(self, preprint, validated_data):
                 detail='Cannot provide data links when has_data_links is set to "no".',
             )
 
-        if updated_has_prereg_links != 'no' and updated_why_no_prereg:
+        if updated_has_prereg_links != 'no' and (updated_why_no_prereg and not prereg_links):
             raise exceptions.ValidationError(
                 detail='You cannot edit this statement while your prereg links availability is set to true or is unanswered.',
             )