There's a difference between null content and no content.
If you omit the column name "EXTENTION_XML" from your load statement, then one of two things will happen:
A) If EXTENTION_XML was not given the NOT NULL constraint when the table was created, then its value would be set to null.
B) Otherwise, the statement should be rejected since NOT NULL means that you MUST provide either an explicit or default value for EXTENTION_XML.
That's the normal case, but it's obvious that Oracle is XML aware for this column, and XML requires something like this at a minumum:
Which means that a "no content" string (empty or blank string), would be illegal XML and be rejected. It's even possible that Oracle would consider even a null value to be illegal XML, but without checking documentation or running a test, I cannot confirm that.
An IDE is no substitute for an Intelligent Developer.