Zabbix

A......... [DEV-4649] improved validation of groupBy parameter for event.get API method

* commit 'd3c3cef881acf5e98f1d8aba50637ffc0a336c7f':
  A......... [DEV-4649] minor code simplification during the review
  A......... [DEV-4649] minor code simplification during the review
  A......... [DEV-4649] dropped support for undocumented NULL as a value for the groupBy option
  A......... [DEV-4649] refactored code that is responsible for sorting in *.get API methods
  A......... [DEV-4649] improved code style
  A......... [DEV-4649] improved code style
  A......... [DEV-4649] improved code style
  A......... [DEV-4649] returned str_in_array() to applyQuerySortOptions()
  A......... [DEV-4649] improved groupBy validation in applyQuerySortOptions()
  A......... [DEV-4649] improved groupBy validation in applyQuerySortOptions()