Replace PHP7-only operators with their older counterparts

Problem/Motivation

There are a few places in code where coalescence operators are used.

ConditionalFieldEditForm.php, line 332

    $selected_state = $form_state->getValue('state') ?: $condition['settings']['state'];

ConditionalFieldEditForm.php, line 190

    '#default_value' => $settings['regex'] ?? '',

We have to take into consideration the fact that PHP-7 is not mandatory for D8 installs.
This syntax is not valid on older versions of PHP and will throw a Syntax error.

Proposed resolution

Replace the mentioned operators with their older counter-parts, according to: https://secure.php.net/manual/en/migration70.new-features.php