TRUNK-4307-Fix: Refactored code as recommended.

https://issues.openmrs.org/browse/TRUNK-4307

Added the following changes:
- Throw APIException in ImplementationIdValidator.validate method when ImplementationId object is null
- Javadoc to ImplementationIdValidatorTest.java class.
@dkayiwa used this approach so I could be able to check the error message