Fix activity.case_id token to always display the first case ID

[mattwire]

Overview

activity.case_id is always an array because it is possible to have an activity linked to multiple cases.

Before

{activity.case_id} token displays nothing because it is an array.

After

We take the first case_id from the array and display it as {activity.case_id}

Technical Details

Comments

[civibot]

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[demeritcowboy]

• General standards
  • [r-explain] PASS
    • Can reproduce and I get the following when moving on the next page: Warning: htmlentities() expects parameter 1 to be string, array given in Civi\Token\TokenRow->fill() (line 236 of blah/Civi/Token/TokenRow.php).
  • [r-user] PASS
  • [r-doc] PASS
  • [r-run] PASS Works.
• Developer standards
  • [r-tech] PASS
    • I have some stream-of-consciousness thoughts which I'll put in a lab ticket but which shouldn't hold up this PR. I think it would also apply to the other tokens PR WIP Activity pdf 71 rebased #16200 because it's the same question of how to deal with a multivalued field like activity.target_contact in tokens.
  • [r-code] PASS
  • [r-maint] PASS
  • [r-test] PASS

[demeritcowboy]

For further discussion ticket: https://lab.civicrm.org/dev/core/issues/1516

[seamuslee001]

Merging based on @demeritcowboy 's review