Avoid calling DataProvider::size when it's not needed (#230)

Now it is not called when using a lazy data provider, because
- the size is used to check if client-side filtering can be used
- lazy data providers always handle filtering in server-side anyway

Fix #222

vaadin-combo-box-flow/src/main/java/com/vaadin/flow/component/combobox/ComboBox.java

@@ -520,9 +520,8 @@ public <C> void setDataProvider(DataProvider<T, C> dataProvider,
     }
 
     private void refreshAllData(boolean forceServerSideFiltering) {
-        int size = getDataProvider().size(new Query<>());
-        setClientSideFilter(
-                !forceServerSideFiltering && size <= getPageSizeDouble());
+        setClientSideFilter(!forceServerSideFiltering && getDataProvider()
+                .size(new Query<>()) <= getPageSizeDouble());
 
         reset();
     }