Rename field validation methods in TransformationUtility class

[fabiocarvalho777]

In TransformationUtility class, rename:

• checkForBlankString to validateMandatoryString
• checkForEmptyString to validateOptionalString
• checkForNull to validateMandatoryObject

Also, validateOptionalString method should return a String, which must be:

• null, if the property value is an empty String or null
• the actual value, if the property value is not null neither an empty String