mb_strtolower(): Passing null to parameter #1 ($string) of type string is deprecated

Patch works, thanks

RTBC +1 Thank you.

The steps to reproduce are missing. What did you do to get that error message?

   $originalKeys = $query->getOriginalKeys();
-  $lowerOriginalKeys = mb_strtolower($originalKeys);
+  $lowerOriginalKeys = mb_strtolower($originalKeys ?? '');

Drupal core does not implement any getOriginalKeys() method.
I do not know which method that code is supposed to call, but the reason for $lowerOriginalKeys to be NULL is that it is initialized with a call to a non-existing method.

I found it: It is the Drupal\search_api\Query\QueryInterface::getOriginalKeys() method, which is documented to either return an array, a string, or NULL. The caller should not assume it returns a string.

Still, in a bug report, the title and quoting an error/exception are not sufficient. A bug report should also describe the steps necessary to reproduce the bug.

I will work on this.

Please review MR.