[INTERNAL] JSDoc: make interface validation more robust
[INTERNAL] JSDoc: make interface validation more robust

The validation of an implementation against an implemented interface
should not crash when the implementation has no methods at all.

Change-Id: I9ced10f52f03ea66e81e981f0855f1aa85665576

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