add: contract create serializer, TODO: fix errors with owners create and test

core/apps/companies/models/folders.py

@@ -37,6 +37,8 @@ class CompanyFolderModel(UUIDPrimaryKeyBaseModel):
         ContractModel,
         verbose_name=_("Contracts"),
         related_name="folders",
+        null=True,
+        blank=True
     )
 
     def __str__(self):