Ensure safe discussion updates

[mnkiefer]

• Reported discussion update test showed a bug in safe outputs that only try to update discussion labels:

safe-outputs:
  update-discussion:
    target: "*"
    allowed-labels:
      - Label1
      - Label2 

• Which became:
  safeoutputs-update_discussion(body: {"add_labels": ["Label3", "Label4"]}, discussion_number: 123)

• This test lead to an updated issue body with add_labels... content which is wrong. It should have only updated the labels and not touched the body at all.