`TernaryToNullCoalescingFixer` - support `isset` in parentheses

Feature request

It would be great if ternary_to_null_coalescing rule treated this:

(isset($test)) ? $test : ''

the same way as this:

isset($test) ? $test : ''

and turn both samples into this:

$test ?? ''

Currently, the first variant stays untouched.

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