Fix BaseXCom.get_all to return empty list instead of None

[shiavm006]

Description

fixes BaseXCom.get_all() to return an empty list instead of None when no XCom values are found, ensuring the implementation matches the documented behavior. the BaseXCom.get_all() method's docstring explicitly states:

"XComSequenceSliceResult can never have None in it, it returns an empty list if no values were found."

the implementation returned None when msg.root was empty, which: Contradicted the documented behavior,Violated type system expectations (XComSequenceSliceResult.root: list[JsonValue])

i can be wrong pls tell me the changes if there is something needed.