Improve contract in executeIf and executeUnless

[fabiocarvalho777]

The contract (API, javadoc and validation) in executeIf and executeUnless, under TransformationUtility, have to be improved.

Here are the rules:

1. It must accept ONLY booleans
2. It must declare clearly what happens if a non boolean value is passed
3. It must declare clearly what happens if a null is passed
4. It must have built-in validation to check non booleans or null values