Fixes #179
Fixes #179

When the keyArray is empty the code tried to access keyArray.length-1 which resulted in a redundant "undefined"
entry being added to the data top level when "return empty fields" is set.
dschnelldavis/angular2-json-schema-form,dschnelldavis/angular2-json-schema-form,dschnelldavis/angular2-json-schema-form