@Nullable
    private static String toScopeString(@Nullable List<String> scopeList) {
        if (scopeList == null || scopeList.isEmpty()) {
            return null;
        }
        return String.join(SCOPE_SEPARATOR, scopeList);
    }
Composes space-separated list of scopes as a {@code JSON} string.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/OAuth2TokenDescriptor.java