Fix potential NPE in seinterpreter eval function

According to the Selenium javadoc: http://selenium.googlecode.com/git/docs/api/java/org/openqa/selenium/remote/RemoteWebDriver.html RemoteWebDriver.eval() can return null if the evaluated javascript does not contain a return statement.  However the current seinterpreter always calls the .toString method on the eval result without doing a null check first, which will result in a NullPointerException when the result is null.  I simply added in a null check here to make sure this doesn't happen.