fix: improper failure if using an empty service definition in ``compose.yml``

An empty definition is useful if you need the service to be selected as a
default service when using ``compose up`` with no argument, and you don't
have anything to configure in the given service.

bin/compose

@@ -987,6 +987,7 @@ _get_compose_service_def_cached () {
         return 0
     fi
     value=$(echo "$docker_compose" | shyaml get-value "$service" 2>/dev/null)
+    [ "$value" == None ] && value=""
     if ! echo "$value" | shyaml get-value "charm" >/dev/null 2>&1; then
         if charm.exists "$service"; then
             value=$(merge_yaml <(echo "charm: $service") <(echo "$value")) || {
@@ -2094,6 +2095,7 @@ _get_docker_compose_mixin_from_metadata_cached() {
     fi
 
     image=$(echo "$metadata" | shyaml get-value "docker-image" 2>/dev/null)
+    [ "$image" == "None" ] && image=""
     image_or_build_statement=
     if [ "$image" ]; then
         if [ "$subordinate" ]; then