Calendar: Fix warning about Undefined variable $firstbookableday

[datengraben]

Fixes a warning about undefined variable $firstbookableday in commonsbooking/src/View/Calendar.php (Line: 399)

Error code: E_WARNING
[Message]:
Undefined variable $firstbookableday

@hansmorb I don't really at the moment if null is a problem here for the JS client code, but it was null for calendars with no timeframes (see the if and the count in Line 363/364) in the past anyways, so I figured it should work this way.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.23%. Comparing base (4841327) to head (041b169).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1639   +/-   ##
=========================================
  Coverage     50.23%   50.23%           
  Complexity     2722     2722           
=========================================
  Files            99       99           
  Lines         10867    10868    +1     
=========================================
+ Hits           5459     5460    +1     
  Misses         5408     5408           

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