[wlan] Initialize empty arrays in device query

Since the FIDL representation of our device query response does
not allow for nullable arrays, we need to initialize them to be
empty in case no values are provided by the driver.

NET-502

Change-Id: I8e345327566d2dfc3404c9c912fb3cec90203701
diff --git a/drivers/wlan/wlan/dispatcher.cpp b/drivers/wlan/wlan/dispatcher.cpp
index b8aa793..ac074ba 100644
--- a/drivers/wlan/wlan/dispatcher.cpp
+++ b/drivers/wlan/wlan/dispatcher.cpp
@@ -495,6 +495,8 @@
     ZX_DEBUG_ASSERT(method == Method::DEVICE_QUERY_request);
 
     auto resp = DeviceQueryResponse::New();
+    resp->modes.resize(0);
+    resp->bands.resize(0);
     const wlanmac_info_t& info = device_->GetWlanInfo();
     if (info.mac_modes & WLAN_MAC_MODE_STA) { resp->modes.push_back(MacMode::STA); }
     if (info.mac_modes & WLAN_MAC_MODE_AP) { resp->modes.push_back(MacMode::AP); }