[Test] Demonstrate the truthiness of CRM_Utils_System::isNull

[demeritcowboy]

Overview

I'm not suggesting to change anything since it's been around since 1.x. This is just based on a footnote from https://lab.civicrm.org/dev/core/issues/937#note_17036 and I couldn't find any tests for it.

Technical Details

I can add some details if required, but the test seems self-explanatory.

[civibot]

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[colemanw]

The function is so screwy it should probably be marked @deprecated.
Php already has isset() which IMO is the preferable way to test if a variable is either null or undefined.