TMDM-7396: "Error while modify Polymorphic FK with row editor"

Fixes issue in web UI data model in foreign key info return (empty foreign key info should never exist).
(https://jira.talendforge.org/browse/TMDM-7396)
TMDM-7396: "Error while modify Polymorphic FK with row editor"

Fixes use of StringUtils in TypeModel (not present for GWT compilation)
(https://jira.talendforge.org/browse/TMDM-7396)
Merge branch 'maintenance/5.6'

Conflicts:
	org.talend.mdm.webapp.base/src/main/java/org/talend/mdm/webapp/base/shared/TypeModel.java