[FIX] sap.m.OverflowToolbar: No exception when content is undefined
[FIX] sap.m.OverflowToolbar: No exception when content is undefined

Problem: When there is undefined as a conent in OverflowToolbar, moving
the controls in suitable collections throws an exception.

Solution: Check for truthy value before performing actions with
controls.

Change-Id: Ib748a1398cc0085de3d479cf07dc574c9f0bd559
BCP: 1970454120

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5