JIT isn't available in PHP <8.0

[cpeel]

JIT isn't available in PHP <8.0 and accessing the jit key in $status results in a Notice:

Notice: Undefined index: jit in /path/opcache-gui/index.php on line 499

Notice: Trying to access array offset on value of type null in /path/opcache-gui/index.php on line 499

The code handles this scenario, we just need to squash the lack of the key causing the notice.

Fixes #114