[REF][PHP8.1] Don't pass null to strtoupper in getDynamicCharacters

[braders]

Overview

Don't pass null to strtoupper in CRM_Utils_PagerAToZ::getDynamicCharacters

Before

Mailings (and possibly other sortable entities) are allowed to be saved without a name. When visiting the "Find mailings" page, this error was triggered for each row without a name:

Deprecated function: strtoupper(): Passing null to parameter #1 ($string) of type string is deprecated in CRM_Utils_PagerAToZ::getDynamicCharacters() (line 100 of [...]/web/sites/all/modules/civicrm/CRM/Utils/PagerAToZ.php).

After

We pass an empty string instead of null to keep PHP happy.

[civibot]

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