fix: default value for ``auto`` in relation definition of ``metadata.yml`` enforced on empty values.

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

bin/compose-core

@@ -2568,6 +2568,7 @@ get_all_relations () {
             fi
 
             auto=$(echo "$relation_def" | shyaml get-value auto pair 2>/dev/null)
+            auto=${auto:-pair}
             case "$auto" in
                 "pair"|"summon")
                     service_list=()
@@ -2602,7 +2603,7 @@ get_all_relations () {
                         fi
                     fi
                     ;;
-                ""|null|disable|disabled)
+                null|disable|disabled)
                     :
                     ;;
                 *)