fix: position packet `want_response` handling

Process valid position broadcasts with `want_response` true except from position requests with an empty payload (latitude and longitude are zeroes).

app/src/main/java/com/geeksville/mesh/service/MeshService.kt

@@ -678,11 +678,14 @@ class MeshService : Service(), Logging {
 
                     // Handle new style position info
                     Portnums.PortNum.POSITION_APP_VALUE -> {
-                        if (data.wantResponse) return // ignore data from position requests
                         val u = MeshProtos.Position.parseFrom(data.payload)
                         // debug("position_app ${packet.from} ${u.toOneLineString()}")
+                        if (data.wantResponse && u.latitudeI == 0 && u.longitudeI == 0) {
+                            debug("Ignoring nop position update from position request")
+                        } else {
                             handleReceivedPosition(packet.from, u, dataPacket.time)
                         }
+                    }
 
                     // Handle new style user info
                     Portnums.PortNum.NODEINFO_APP_VALUE ->