ExternalProject: No svn --username if empty (#11173)

Nor --password. But please do if non-empty. Thank you.

Modules/ExternalProject.cmake

@@ -985,8 +985,15 @@ function(_ep_add_download_command name)
     get_filename_component(src_name "${source_dir}" NAME)
     get_filename_component(work_dir "${source_dir}" PATH)
     set(comment "Performing download step (SVN checkout) for '${name}'")
+    set(svn_user_pw_args "")
+    if(svn_username)
+      set(svn_user_pw_args ${svn_user_pw_args} "--username=${svn_username}")
+    endif()
+    if(svn_password)
+      set(svn_user_pw_args ${svn_user_pw_args} "--password=${svn_password}")
+    endif()
     set(cmd ${Subversion_SVN_EXECUTABLE} co ${svn_repository} ${svn_revision}
-      --username=${svn_username} --password=${svn_password} ${src_name})
+      ${svn_user_pw_args} ${src_name})
     list(APPEND depends ${stamp_dir}/${name}-svninfo.txt)
   elseif(git_repository)
     find_package(Git)
@@ -1125,8 +1132,15 @@ function(_ep_add_update_command name)
     get_property(svn_revision TARGET ${name} PROPERTY _EP_SVN_REVISION)
     get_property(svn_username TARGET ${name} PROPERTY _EP_SVN_USERNAME)
     get_property(svn_password TARGET ${name} PROPERTY _EP_SVN_PASSWORD)
+    set(svn_user_pw_args "")
+    if(svn_username)
+      set(svn_user_pw_args ${svn_user_pw_args} "--username=${svn_username}")
+    endif()
+    if(svn_password)
+      set(svn_user_pw_args ${svn_user_pw_args} "--password=${svn_password}")
+    endif()
     set(cmd ${Subversion_SVN_EXECUTABLE} up ${svn_revision}
-      --username=${svn_username} --password=${svn_password})
+      ${svn_user_pw_args})
     set(always 1)
   elseif(git_repository)
     if(NOT GIT_EXECUTABLE)