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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/adamspd/django-appointment/pull/166/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Adams+Pierre+David) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/adamspd/django-appointment/pull/166/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Adams+Pierre+David) | `95.34% <100.00%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Adams+Pierre+David#carryforward-flags-in-the-pull-request-comment) to find out more.

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