build: package-snap: Default SNAP_PKG_NAME to 'jami'.

This variable is set on the CI; it's nice to have it default to
something rather than fail.

* scripts/make-packaging-target.py (run_generate_all): Expand
SNAP_PKG_NAME to 'jami' if unset or null.

Change-Id: Iacc6edd62388c0e803928863e6e53ddd7314379a