Code Modernization: replace if isset() return with null coalescing

[Soean]

In #63430 isset() ternary were replaced with null coalescing.

We also should replace if isset() return with null coalescing.

<?php
-        if (isset($this->items[$key])) {
-            return $this->items[$key];
-        }
-
-        return 'fallback value';
+        return $this->items[$key] ?? 'fallback value';

This is part of #58874.

Trac ticket: https://core.trac.wordpress.org/ticket/64488

@Soean I found some additional opportunities in de042b59f67db0d6014be62bbe83070962ce8ad1. Take a look.

[mukesh27]

Mark ready for commit.

@westonruter will commit this soon.

[westonruter]

In 61470:

Code Modernization: Replace if statements with null coalescing operator.

Developed in ​https://github.com/WordPress/wordpress-develop/pull/10703

Follow-up to [61464], [61463], [61457], [61456], [61455], [61454], [61453], [61445], [61444], [61443], [61442], [61436], [61435], [61434], [61403], [61433], [61432], [61431], [61430], [61429], [61424], [61404], [61403].

Props soean, westonruter, mukesh27.
See #58874.
Fixes #64488.