Make sure format() returns string.

[jonpugh]

I am getting a strange failure when testing some old Drupal code:

TypeError: Behat\Behat\Output\Printer\Formatter\ConsoleFormatter::format(): Return value must be of type string, null returned in Behat\Behat\Output\Printer\Formatter\ConsoleFormatter->format() (line 33 of /usr/share/devshop/src/DevShop/Control/vendor/behat/behat/src/Behat/Behat/Output/Printer/Formatter/ConsoleFormatter.php).

Looking into it, I discovered format() calls preg_replace_callback, which returns null|string|string[].

This change returns an empty string if preg_replace_callback returns null.

Should we check for an array as well?

[stof]

Can you share a reproducer for a case making the regex fail ?

[stof]

Should we check for an array as well?

No. preg_replace_callback returns an array when its subject argument is an array, which is not the case here.

[mdio]

I ran into this when the response body (which was checked) was generated by https://github.com/filp/whoops due to an exception being thrown. Unfortunately I cannot share the content because it's full of internal information.
The HTML was 1 MB large with a 6 level stack trace and roughly a thousand environment variables.
The error for preg_replace_callback was PREG_BACKTRACK_LIMIT_ERROR with pcre.backtrack_limit being 1000000 (the default).

I believe issue #1431 is about the same problem.

Maybe this helps to craft a test case.