ExternalProject: Don't add SVN auth args if not set

In 462e583267 (ExternalProject: Switch download, update and patch to
use _EP_ vars, 2024-05-13), the svn_username and svn_password variables
were only updated appropriately in the svn download implementation, but
not in the svn update implementation. This was easily missed in part
because the old implementation was relying on behavior that differed
from the documentation for get_property(), and the new implementation
tried to keep the intermediate variables from the old implementation but
missed this subtle aspect.

Issue: #26152
Fixes: #26148

Modules/ExternalProject/shared_internal_commands.cmake

@@ -1495,10 +1495,10 @@ function(_ep_add_update_command name)
       set(svn_interactive_args "--non-interactive")
     endif()
     set(svn_user_pw_args "")
-    if(DEFINED svn_username)
+    if(DEFINED _EP_SVN_USERNAME)
       set(svn_user_pw_args ${svn_user_pw_args} "--username=${svn_username}")
     endif()
-    if(DEFINED svn_password)
+    if(DEFINED _EP_SVN_PASSWORD)
       set(svn_user_pw_args ${svn_user_pw_args} "--password=${svn_password}")
     endif()
     if(svn_trust_cert)