[fix] Added "default" kwarg for unique nullable fields

Signed-off-by: DragnEmperor <[email protected]>

Author: DragnEmperor

openwisp_controller/pki/api/serializers.py

@@ -73,7 +73,12 @@ class Meta:
         ]
         read_only_fields = ["created", "modified"]
         extra_kwargs = {
-            "organization": {"required": True},
+            # In DRF 3.16+, nullable fields that are part of a unique constraint
+            # automatically get `default: None`, which can cause validation issues.
+            # Setting the default to `serializers.empty` ensures DRF does not treat
+            # these fields as both required and having a default value, avoiding
+            # conflicts.
+            "organization": {"required": True, "default": serializers.empty},
             "common_name": {"default": "", "required": False},
             "key_length": {"initial": "2048"},
             "digest": {"initial": "sha256"},