Unified workflow with empty string and null in setAttributes and setAttribute
 - Added method convertBlankStringIntoNullInAttrValue into AttributesManagerBl
 - Modified methods setAttribute and setAttributes in AttributeManagerEntry
   - convertBlankStringIntoNullInAttrValue convert empty string into null for checkAttributeValue and checkAttributeExists() to unify work with empty string and null
 - Some database are not able to recognize empty string and null. This is the reason why we convert empty string into null in entry.
Merge pull request #229 from papperwing/BlankString

Unified workflow with empty string and null in setAttributes and setAttribute