Code Modernization: Use null coalescing operator in WP_Block_Parser class methods

[mukeshpanchal27]

Trac ticket: https://core.trac.wordpress.org/ticket/65818

GB WordPress/gutenberg#82763

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[t-hamano]

Thanks for the PR. We should update here as well.

wordpress-develop/src/wp-includes/class-wp-block-parser.php

Lines 326 to 327 in 5b1696e

	public function add_freeform( $length = null ) {
	$length = $length ? $length : strlen( $this->document ) - $this->offset;

Additionally, could you submit a Gutenberg PR? There should be similar sections.

[t-hamano]

Committed: https://core.trac.wordpress.org/changeset/63601