static boolean isUndefined(EndpointGroup endpointGroup) {
        return UndefinedEndpointGroup.of() == endpointGroup;
    }
Returns {@code true} if the specified {@code endpointGroup} is an undefined {@link EndpointGroup},
which signifies that a request was created without a {@link URI} or {@link EndpointGroup}.
For example,
<pre>{@code
HttpPreprocessor preprocessor = (delegate, ctx, req) -> {
    if (EndpointGroup.isUndefined(ctx.endpointGroup())) {
        ctx.setEndpointGroup(Endpoint.of("fallback-endpoint"));
    }
    return delegate.execute(ctx, req);
};
WebClient client = WebClient.builder(preprocessor)
                            .build();
}</pre>
core/src/main/java/com/linecorp/armeria/client/endpoint/EndpointGroup.java