[PHPBB3-16883] Check if var is array before using count in installer diff

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Running installer "Update files" leads to an exception in diff.php:

PHP Fatal error:  Uncaught TypeError: count(): Argument #1 ($var) must be of type Countable|array, bool given in phpbb/includes/diff/diff.php:172

The test in is_empty assumes $edit->orig and $edit->final are always arrays, but they're booleans in some cases:

class diff_op_delete extends diff_op

	function __construct($lines)

		$this->orig = $lines;

		$this->final = false;

	function &reverse()

		$reverse = new diff_op_add($this->orig);

		return $reverse;

class diff_op_add extends diff_op

	function __construct($lines)

		$this->final = $lines;

		$this->orig = false;

	function &reverse()

		$reverse = new diff_op_delete($this->final);

		return $reverse;

I'm not familiar with this code, but a workaround might be adding a test of is_array.