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

Fixes https://github.com/SAP/openui5/issues/239 

Change-Id: I42c6d88acaa0fd6a4d71a0a3f3503db910f47968

SAP/openui5,SQCLabs/openui5,cschuff/openui5,olirogers/openui5,SQCLabs/openui5,nzamani/openui5,cschuff/openui5,SAP/openui5,SAP/openui5,SQCLabs/openui5,cschuff/openui5,SAP/openui5,nzamani/openui5,olirogers/openui5,nzamani/openui5,olirogers/openui5