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

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

bin/compose-core

@@ -2611,7 +2611,8 @@ get_all_relations () {
                     return 1
                     ;;
             esac
-            constraint=$(echo "$relation_def" | shyaml get-value constraint optional 2>/dev/null)
+            constraint=$(echo "$relation_def" | shyaml get-value constraint 2>/dev/null)
+            constraint=${constraint:-optional}
             case "$constraint" in
                 "required")
                     required+=("$service" "$relation_name" "$relation_def")