Don't write config/seeded-snaps if no snaps have been seeded.

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.498) UNRELEASED; urgency=medium
+
+  * Don't write config/seeded-snaps if no snaps have been seeded.
+
+ -- Iain Lane <iain.lane@canonical.com>  Thu, 01 Feb 2018 11:52:16 +0000
+
 livecd-rootfs (2.497) bionic; urgency=medium
 
   * live-build/auto/config: Clean out unused hostname matches and fix indenting.

live-build/auto/config

@@ -676,8 +676,10 @@ if [ -n "${BASE_SEED}" ]; then
 			ALL_SNAPS="${ALL_SNAPS:+${ALL_SNAPS} }${snap}"
 		done
 	done
+	if [ -n "${ALL_SNAPS}" ]; then
 		echo "${ALL_SNAPS}" > config/seeded-snaps
 	fi
+fi
 
 export APT_OPTIONS