RFR 8072595: nashorn should not use obj.getClass() for null checks

[A. Sundararajan]

Please review http://cr.openjdk.java.net/~sundar/8072595/ for 
https://bugs.openjdk.java.net/browse/JDK-8072595

See also: http://cr.openjdk.java.net/~shade/scratch/NullChecks.java
PS. Piggybacking a script engine test (which fails in 8u31 but passes in 
8u40 - not sure if we had a test to cover it and hence adding it) and 
couple of minor issues with sample code as well.

Thanks,
-Sundar