Handle path parsing for opaque URIs

[AlexanderEllis]

When we're working with an opaque URI that the netty code can't parse a path from, we can end up with a null path, causing a NPE as we try to parse it.

This adds a quick check — if we don't have a path, just return the URI. This matches the existing behavior — scope out the parseUriFromNetty_unknown test:

    @Test
    void parseUriFromNetty_unknown() {

        EmbeddedChannel channel = new EmbeddedChannel(new ClientRequestReceiver(null));
        channel.attr(SourceAddressChannelHandler.ATTR_SERVER_LOCAL_PORT).set(1234);
        HttpRequestMessageImpl result;
        {
            channel.writeInbound(
                    new DefaultFullHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.POST, "asdf", Unpooled.buffer()));
            result = channel.readInbound();
            result.disposeBufferedBody();
        }

        assertThat(result.getPath()).isEqualTo("asdf");

        channel.close();
    }

We can match this behavior with our null check defense.