[FIX] MessagePage: Fixed message page rule
[FIX] MessagePage: Fixed message page rule

Added additional issue check which ensures
that MessagePage is not the first child of the UI Area.

Since the MessagePage can exist without siblings,
the rule omits the cases where MessagePage is
the only child of the UI Area.

Change-Id: Iff48eaff8e3bdbab5f756aa09e61491dc236c767

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