ExternalProject: Allow blank SVN_USERNAME/SVN_PASSWORD (#14128)

With

  SVN_USERNAME ""
  SVN_PASSWORD ""

in an ExternalProject_Add() call, the blank username and password
will be passed to the svn checkout/update step commands.
Merge topic 'ExternalProject-svn-auth-blank' into next

ecd11a2 ExternalProject: Allow blank SVN_USERNAME/SVN_PASSWORD (#14128)