Assert temp output directory is not an empty string

Signed-off-by: voloder <110066198+voloder@users.noreply.github.com>

Makefile

@@ -250,6 +250,9 @@ swagger-docs: ## preview the API documentation
 .PHONY: generate-files
 generate-files:
 	$(eval $@_TMP_OUT := $(shell mktemp -d -t moby-output.XXXXXXXXXX))
+	ifeq ($($@_TMP_OUT),)
+		$(error Could not create temp directory.)
+	endif
 	$(BUILD_CMD) --target "update" \
 		--output "type=local,dest=$($@_TMP_OUT)" \
 		--file "./hack/dockerfiles/generate-files.Dockerfile" .