Fix local variable 'activities' referenced before assignment

[hugovk]

Followup to #10.

From CI:

Traceback (most recent call last):
Plotting landscape...
  File "C:\hostedtoolcache\windows\Python\3.7.9\x64\lib\runpy.py", line 193, in _run_module_as_main
Saved to strava-landscape.png
    "__main__", mod_spec)
  File "C:\hostedtoolcache\windows\Python\3.7.9\x64\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\hostedtoolcache\windows\Python\3.7.9\x64\Scripts\stravavis.exe\__main__.py", line 7, in <module>
  File "C:\hostedtoolcache\windows\Python\3.7.9\x64\lib\site-packages\stravavis\cli.py", line [72](https://github.com/marcusvolz/strava_py/runs/5136794948?check_suite_focus=true#step:5:72), in main
    if activities:
UnboundLocalError: local variable 'activities' referenced before assignment
Error: Process completed with exit code 1.

https://github.com/marcusvolz/strava_py/runs/5136794948?check_suite_focus=true

Need to initialise activities = None in case this if doesn't run:

    activities = None
    if args.activities_path:
        print("Processing activities...")
        activities = process_activities(args.activities_path)

So there's a variable to check here:

    if activities:
        print("Plotting calendar...")
        outfile = f"{args.output_prefix}-calendar.png"
        plot_calendar(activities, output_file=outfile)
        print(f"Saved to {outfile}")