DisplayUser velocity macro is crashing when used with a null value

The macros.vm expose a #displayUser velocity macro that can be used for displaying a user. However this macro seems to not support providing a null value, which is a common usecase with Guest user, since it's internally stored as null.
Moreover, the interpretation of an empty string is a bit surprising. IMO it should resolve both null and empty string to display Guest user information.

Reproduction steps:

• Create a page with:

  {{velocity}}
  #displayUser()
  {{/velocity}}
  

Expected result:

• the macro displays Guest user

Obtained result:

• a NPE