[FIX] routing.History: clarify return type of getPreviousHash
[FIX] routing.History: clarify return type of getPreviousHash

Closes: https://github.com/SAP/openui5/pull/3517
Change-Id: I2c0f2e9c1c2c83d79afb3082a1c9be12fa2b8df0

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