Scanner should not fail when an issue message has a NULL character

Scanner fails when an issue is generated with a message containing the NULL character. That can occur when the issue message includes source code:

Character \u0000 is not supported in issue message "Refactor this SQL query to prevent doing a full table scan due to the '%%' value of the "LIKE" condition' on component: SOURCE/FOO/bar.cbl`

Making the scanner fail impacts negatively user experience, especially when null characters are supported by compilers (Cobol for instance). The behavior of scanner should be changed by replacing NULL characters \u0000 with the text "[NULL]" (6 characters).

See https://discuss.sonarsource.com/t/cobol-low-values-in-issue-messages/5010/4