UniqueAuthorNameValidator throws NPE when reposoitory alias is added and table author contains {{null}} entries

if there is a null entry in author table for author_name then
java.lang.NullPointerException
at com.atlassian.bamboo.ww2.validators.UniqueAuthorNameValidator.validate(UniqueAuthorNameValidator.java:44)
at com.opensymphony.xwork.validator.DefaultActionValidatorManager.validate(DefaultActionValidatorManager.java:190)
at com.opensymphony.xwork.validator.DefaultActionValidatorManager.validate(DefaultActionValidatorManager.java:97)
at com.opensymphony.xwork.validator.DefaultActionValidatorManager.validate(DefaultActionValidatorManager.java:84)
is thrown
when an alias is added for a user.

the null entry could be caused by a 3rd party macro and we should be more defensive in UniqueAuthorNameValidator#validate