Fix LayoutlibTestCallback.getResourceId

Return null instead of triggering a NPE when there are no resource of
the asked type.

Test: RenderTests.testFonts
Change-Id: Ib45ebdf2178e62cbd987082512fcbb009de3f1b2

tools/layoutlib/bridge/tests/src/com/android/layoutlib/bridge/intensive/setup/LayoutLibTestCallback.java

@@ -132,7 +132,11 @@ public class LayoutLibTestCallback extends LayoutlibCallback {
 
     @Override
     public Integer getResourceId(ResourceType type, String name) {
-        return mResources.get(type).get(name);
+        Map<String, Integer> resName2Id = mResources.get(type);
+        if (resName2Id == null) {
+            return null;
+        }
+        return resName2Id.get(name);
     }
 
     @Override