appointment.views: some improvements

[deronnax]

• the staff_member of the form mandatorily exist (the form guarantees it through it is_valid method), so the whole if not block can be removed
• a None variable and an empty string are both falsy in Python, you can check for both in the same time with just if xxx

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.34%. Comparing base (db269f3) to head (0df4d22).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
+ Coverage   95.33%   95.34%   +0.01%     
==========================================
  Files          50       50              
  Lines        5784     5779       -5     
==========================================
- Hits         5514     5510       -4     
+ Misses        270      269       -1     

Flag	Coverage Δ
unittests	95.34% <100.00%> (+0.01%)	⬆️

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