pull: Avoid "Migrating profile" message on the first run.

* guix/scripts/pull.scm (ensure-default-profile): Do not call
'migrate-generations' when
%USER-PROFILE-DIRECTORY (~/.config/guix/current) does not exist.  This
avoids a confusing "Migrating profile" message when the user runs 'guix
pull' for the first time.

guix/scripts/pull.scm

@@ -507,6 +507,7 @@ true, display what would be built without actually building it."
   ;; workaround, skip this code when $SUDO_USER is set.  See
   ;; <https://bugs.gnu.org/36785>.
   (unless (or (getenv "SUDO_USER")
+              (not (file-exists? %user-profile-directory))
               (string=? %profile-directory
                         (dirname
                          (canonicalize-profile %user-profile-directory))))