VR HWC: Relax checks around layer presentation

Rather than error out on layers without buffers, simply discard them.
This is more in line with other HWComposer implementations which deal
with this error, since SurfaceFlinger doesn't handle it appropriately.

Bug: 37091090
Test: Ran on device and verified errors are no longer reported in logcat
Change-Id: Iacfd9fa707d86367c7b6dbf62edea297fc9dfb8e
diff --git a/services/vr/vr_window_manager/composer/impl/vr_hwc.cpp b/services/vr/vr_window_manager/composer/impl/vr_hwc.cpp
index d142729..de7756c 100644
--- a/services/vr/vr_window_manager/composer/impl/vr_hwc.cpp
+++ b/services/vr/vr_window_manager/composer/impl/vr_hwc.cpp
@@ -217,19 +217,14 @@
       queued_client_target = true;
     } else {
       if (!layer.info.buffer.get() || !layer.info.fence.get()) {
-        ALOGE("Layer requested without valid buffer");
-        return Error::BAD_LAYER;
+        ALOGV("Layer requested without valid buffer");
+        continue;
       }
 
       frame.push_back(layer.info);
     }
   }
 
-  if (frame.empty()) {
-    ALOGE("Requested frame with no layers");
-    return Error::BAD_LAYER;
-  }
-
   out_frames->swap(frame);
   return Error::NONE;
 }