[FIX] ManagedObject: In case of a format exception set a null value


Fixes https://github.com/SAP/openui5/issues/239 
Change-Id: I11bf5cea3caad55a83ddd6c3817b28fb373aca35
CR-Id: 012006153200015414772014
BCP: 1472003897
Merge "[FIX] ManagedObject: In case of a format exception set a null value" into rel-1.26