static code analysis fixes

Models/Calendar.php

@@ -110,7 +110,7 @@ class Calendar
         $key = \key($this->events);
         \reset($this->events);
 
-        return $key ?? 0;
+        return $key;
     }
 
     /**

composer.json

@@ -12,7 +12,7 @@
         "friendsofphp/php-cs-fixer": ">=3.2",
         "squizlabs/php_codesniffer": ">=3.6",
         "phpmd/phpmd": ">=2.9",
-        "phpstan/phpstan": ">=0.12.58",
+        "phpstan/phpstan": ">=1.5.4",
         "phan/phan": ">=3.2.6"
     },
     "minimum-stability": "dev",