fix(typeahead): typeahead prepareMatches method doesn't check for undefined observables

[danielGz]

I noticed that prepareMatches doesn't check when options = undefined, in cases when you're using an observable as input parameter to the directive and the observable returns a null object (such as when getting a 204 NO CONTENT from a service) the module crashes. Maybe there's a work around that doesn't involve modifying the directive code but I wanted to check first.

[valorkin]

Nope, it is an issue to fix,
As a workaround always return something from Observables