DateParser: Fix "str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated"

[paladox]

Deprecated: str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in /var/www/html/extensions/SemanticResultFormats/src/iCalendar/DateParser.php on line 33

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.51%. Comparing base (ace37f7) to head (374ba70).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #895   +/-   ##
=========================================
  Coverage     45.51%   45.51%           
  Complexity     2319     2319           
=========================================
  Files            80       80           
  Lines          9043     9043           
=========================================
  Hits           4116     4116           
  Misses         4927     4927           

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