[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
(cherry picked from commit c2c751624f7c67e9dd96f8dde792776944a9d170)
Merge pull request #2031 from sqliu/crosswalk-6

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