Add check to non-empty pattern in `HttpHeader::withPattern()` method

[vjik]

Do we need to add such a check and throw exception if pattern empty?

[rustamwin]

Either we need to check the string or remove the non-empty string annotation.

[vjik]

Either we need to check the string or remove the non-empty string annotation.

Pattern is used in preg_match (https://github.com/yiisoft/auth/blob/master/src/Method/HttpHeader.php#L100) and throws warning when it is empty. I think, non-empty string annotation must be keep.