It's possible of use a setter with a find. I will like replace content of field 'aboutMe' if it's empty.

demo.php

object(Cake\ORM\Entity) {

	'user_id' => (int) 1,
	'username' => 'demo100',
	'avatar' => false,
	'password' => '1',
	'email' => '1',
	'token' => null,
	'created' => null,
	'modified' => null,
	'about_me' => '',
	'[new]' => false,
	'[accessible]' => [
		'*' => true
	],
	'[dirty]' => [],
	'[original]' => [],
	'[virtual]' => [],
	'[errors]' => [],
	'[invalid]' => [],
	'[repository]' => 'users'

}