Explicitly guard optional matrix steps in CI workflow

[Copilot]

Steps using optional matrix fields (update, deps1, deps2) relied on GitHub Actions silently no-oping an empty run value — undocumented behavior that works today but is fragile.

Changes

• .github/workflows/ccpp.yml: Added && matrix.config.<field> != '' to the if conditions for update system, install deps (1), and install deps (2) steps, so they are skipped when the field is absent rather than running an empty command.

- name: update system
  if: ${{ matrix.config.enabled == 1 && matrix.config.update != '' }}
  run: ${{ matrix.config.update }}
- name: install deps (2)
  if: ${{ matrix.config.enabled == 1 && matrix.config.deps2 != '' }}
  run: ${{ matrix.config.deps2 }}

Only the MinGW matrix entry defines update and deps2; all other entries omit them and will now cleanly skip those steps.

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[JohannesLorenz]

@copilot you test only against empty strings. Will your solution also work if the matrix entry does not exist?

[Copilot]

@copilot you test only against empty strings. Will your solution also work if the matrix entry does not exist?

Yes — in GitHub Actions expressions, accessing an undefined context property returns an empty string, which is falsy. So both != '' and a truthy check handle the absent-field case.

That said, I've switched to the plain truthy form (matrix.config.update, matrix.config.deps1, matrix.config.deps2) in commit bd548d2, which makes the intent clearer and is idiomatic for optional fields — it works correctly whether the field is absent from the matrix entry or explicitly set to an empty string.