NullPointerException in Blame for line with author == null

In the project of my company we have some files for which blame fails and is unable to retrieve the author of some/all lines. For these files the sonar-scm-activity-plugin throws a NullPointerException in the method normalizeString(String) in the class Blame.
This fix simply adds a null-check in normalizeString(String) and returns an empty String in this case.