fix: prevent null start date in course details (#792)

Author: viadanna

cms/djangoapps/contentstore/xblock_storage_handlers/view_handlers.py

@@ -429,6 +429,11 @@ def _save_xblock(
                 for metadata_key, value in metadata.items():
                     field = xblock.fields[metadata_key]
 
+                    # Prevent setting release date to null
+                    # remove field and inherit from parent instead
+                    if metadata_key == "start" and value == "":
+                        value = None
+
                     if value is None:
                         field.delete_from(xblock)
                     else: