android: BillingManager: Fix issue with onQuerySkuDetailsFinished null param.

- Happens when not associated with a Google account.

src/android/app/src/main/java/org/citra/citra_emu/utils/BillingManager.java

@@ -88,6 +88,10 @@ public class BillingManager implements PurchasesUpdatedListener {
     }
 
     private void onQuerySkuDetailsFinished(List<SkuDetails> skuDetailsList) {
+        if (skuDetailsList == null) {
+            return;
+        }
+
         if (skuDetailsList.isEmpty()) {
             return;
         }