services: Make the urandom-seed-service-type return success properly.

Reported by nigko on #guix:

https://logs.guix.gnu.org/guix/2025-04-05.log#201718

* gnu/services/base.scm (urandom-seed-shepherd-service): Return #f when
stopped.

Change-Id: I8212508e4a017270e4e9284b43170cd17999e8b4

gnu/services/base.scm

@@ -673,7 +673,8 @@ file systems, as well as corresponding @file{/etc/fstab} entries.")))
                              (lambda (seed)
                                (put-bytevector seed buf)))
                            (umask previous-umask))
-                         #t)))))
+                         )))
+                   #f))
          (modules `((rnrs bytevectors)
                     (rnrs io ports)
                     ,@%default-modules)))))