[Android] Fix null callback issue in XWalkView.evaluateJavaScript.

In case that the callback is a `null` value, XWalkView should behave normally and
same as Android WebView.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1815
Merge pull request #2029 from sqliu/fix-evaluate-js

[Android] Fix null callback issue in XWalkView.evaluateJavaScript.