Use PHP 7 '??' operator instead of '?:' with 'isset()' where convenient

Use PHP 7 '??' operator instead of '?:' with 'isset()' where convenient

Find: /isset\(\s*([^()]+?)\s*\)\s*\?\s*\1\s*:\s*/
Replace with: '\1 ?? '

(Everywhere except includes/PHPVersionCheck.php)
(Then, manually fix some line length and indentation issues)

Then manually reviewed the replacements for cases where confusing
operator precedence would result in incorrect results
(fixing those in I478db046a1cc162c6767003ce45c9b56270f3372).

Change-Id: I33b421c8cb11cdd4ce896488c9ff5313f03a38cf