<?php

class CM_I18n_Phrase {

    /** @var $_phrase string */
    protected $_phrase;

    /** @var $_variables string[] */
    protected $_variables;

    /**
     * @param string $phrase
     * @param string[] $variables
     */
    public function __construct($phrase, array $variables = []) {
        $this->_phrase = (string) $phrase;
        $this->_variables = $variables;
    }

    /**
     * @param CM_Frontend_Render $render
     * @return string
     */
    public function translate(CM_Frontend_Render $render) {
        return $render->getTranslation($this->_phrase, $this->_variables);
    }
}

<?php

class CM_I18n_Phrase {

    /** @var $_phrase string */
    protected $_phrase;

    /** @var $_variables string[] */
    protected $_variables;

    /**
     * @param string $phrase
     * @param string[]|null $variables
     */
    public function __construct($phrase, array $variables = null) {
        $this->_phrase = (string) $phrase;
        $this->_variables = is_null($variables) ? [] : $variables;
    }

    /**
     * @param CM_Frontend_Render $render
     * @return string
     */
    public function translate(CM_Frontend_Render $render) {
        return $render->getTranslation($this->_phrase, $this->_variables);
    }
}

Change default type to null
Change default type to null
