avoid deprecation notice if header is null

[jmcclelland]

Overview

I'm getting errors like the following:

[PHP Deprecation] strip_tags(): Passing null to parameter #1 ($string) of type string is deprecated at /var/www/powerbase/sites/all/modules/civicrm/CRM/Report/Utils/Report.php:239

Before

When $form->_columnHeaders[$header] is defined but $form->_columnHeaders[$header]['title'] is not defined or null, then we end up passing NULL to the strip_tags function, which expects a string.

After

By coalescing $form->_columnHeaders[$header]['title'] to a empty string when NULL, we avoid this error.

[civibot]

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