Fix mailer.py broken feature - for_paths and show_nonmatching_paths=yes.
Fix mailer.py broken feature - for_paths and show_nonmatching_paths=yes.

Patch by: Mathias Weinert <mathias.weinert@gfa-net.de>
          me

* tools/hook-scripts/mailer/mailer.py (generate_content):
    Remove comma causing erroneous contruction of tuple.
    Change [ ] to None to avoid giving false impression that a sequence is
    required.

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