Safe output for `update-discussion` warning

[mnkiefer]

Update discussion safe actions with only label updates alone (no title or body updates) do not get applied, but yield the following warnings:

update_discussion requires at least one of: 'title', 'body' fields

Here is an excerpt of the workflow markdown file which is correct:

tools:
  github:
    github-token: ${{ secrets.DISCUSSIONS_TOKEN }}
    toolsets: [discussions, repos]

safe-outputs:
  allowed-github-references: [githubnext/discussion-ops]
  create-issue:
    github-token: ${{ secrets.OPS_ISSUES_TOKEN }}
    title-prefix: "[Daily Labelling Summary]"
    close-older-issues: true
    expires: 7d
  update-discussion:
    github-token: ${{ secrets.DISCUSSIONS_TOKEN }}
    target: "*"
    target-repo: ${{ inputs.target-repo || env.DEFAULT_TARGET_REPOSITORY }}
    max: ${{ inputs.max-discussions || env.DEFAULT_MAX_DISCUSSIONS }}
    labels:
    allowed-labels:
      - label1
      - label2