[stable19] CalDavBackend: check if timerange is array before accessing

[backportbot-nextcloud]

backport of #23497

[faily-bot]

🤖 beep boop beep 🤖

Here are the logs for the failed build:

Status of 34338: failure

acceptance-app-files

• tests/acceptance/features/app-files.feature:108

Show full log

  Scenario: show shares                                               # /drone/src/tests/acceptance/features/app-files.feature:108
    Given I am logged in                                              # LoginPageContext::iAmLoggedIn()
    And I share the link for "welcome.txt"                            # FilesAppSharingContext::iShareTheLinkFor()
    When I open the "Shares" section                                  # AppNavigationContext::iOpenTheSection()
    Then I see that the current section is "Shares"                   # AppNavigationContext::iSeeThatTheCurrentSectionIs()
    Then I see that the file list contains a file named "welcome.txt" # FileListContext::iSeeThatTheFileListContainsAFileNamed()
      Row for file welcome.txt in file list could not be found after 100 seconds (NoSuchElementException)