fix: [compose-core] fail on empty compose file

bin/compose-core

@@ -5616,6 +5616,12 @@ fi >&2
 
 COMPOSE_YML_FILE=$(get_compose_yml_location) || exit 1
 COMPOSE_YML_CONTENT=$(get_compose_yml_content) || exit 1
+
+if [ -z "$COMPOSE_YML_CONTENT" ]; then
+    err "Compose file '$HOST_COMPOSE_YML_FILE' is empty."
+    exit 1
+fi
+
 COMPOSE_YML_CONTENT_HASH=$(compose:yml:hash) || exit 1
 CHARM_STORE_HASH=$(charm.store_metadata_hash) || exit 1
 COMBINED_HASH=$(H "$COMPOSE_YML_CONTENT_HASH" "$CHARM_STORE_HASH") || exit 1