pkg-merge-upstream-to-downstreams: Fix crash when not using --auto-merge

If --auto-merge is not set it results in:

Traceback (most recent call last):
  File "/usr/bin/apertis-pkg-merge-upstream-to-downstreams", line 146, in <module>
    main()
  File "/usr/bin/apertis-pkg-merge-upstream-to-downstreams", line 92, in main
    auto_merge = args.auto_merge.split(":")
AttributeError: 'NoneType' object has no attribute 'split'