Handle path parsing for opaque URIs (#2067)

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.

Author: AlexanderEllis

zuul-core/src/main/java/com/netflix/zuul/netty/server/ClientRequestReceiver.java

@@ -420,7 +420,11 @@ private String parsePath(String uri) {
         try {
             URI uriObject = new URI(uri);
             uriObject = uriObject.normalize();
-            path =  uriObject.getRawPath();
+            path = uriObject.getRawPath();
+            if (path == null) {
+                // If we have an opaque URI, match existing behavior of using the URI as the path.
+                return uri;
+            }
             while (path.startsWith("/..")) {
                 path = path.substring(3);
             }

zuul-core/src/test/java/com/netflix/zuul/netty/server/ClientRequestReceiverTest.java

@@ -536,6 +536,22 @@ void pathTraversal_withQueryString() {
         channel.close();
     }
 
+    @Test
+    void pathTraversal_withOpaqueURI() {
+        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.GET, "foo.netflix.net:443", Unpooled.buffer()));
+            result = channel.readInbound();
+            result.disposeBufferedBody();
+        }
+
+        assertThat(result.getPath()).isEqualTo("foo.netflix.net:443");
+        channel.close();
+    }
+
     @Test
     void pathNormalization_emptyPath() {
         EmbeddedChannel channel = new EmbeddedChannel(new ClientRequestReceiver(null));