Check for value of $this->severity_key prior to stripos

[drupov]

In src/Allure/Behat/Formatter/AllureFormatter.php the check if (stripos($tag, $this->severity_key) === 0) { happens without checking if $this->severity_key has a value. With PHP 7.4 non-string needles are being interpreted as string and of $this->severity_key has e.g. the value of NULL results in an error.

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