Fix: Don't require the JSON user arguments field (#5320)

* Allows new user args field to be null

* Coerce empty string to None for user_args JSONField

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Co-authored-by: shamoon <4887959+shamoon@users.noreply.github.com>

src/paperless/serialisers.py

@@ -122,7 +122,12 @@ class ProfileSerializer(serializers.ModelSerializer):
 
 
 class ApplicationConfigurationSerializer(serializers.ModelSerializer):
-    user_args = serializers.JSONField(binary=True)
+    user_args = serializers.JSONField(binary=True, allow_null=True)
+
+    def run_validation(self, data):
+        if "user_args" in data and data["user_args"] == "":
+            data["user_args"] = None
+        return super().run_validation(data)
 
     class Meta:
         model = ApplicationConfiguration