Adds default $query value on Searchable::search

[nunomaduro]

This Pull Request addresses the developer experience while interacting with the method Searchable::search - adding a default value to the $query param.

This can be useful if the developer just want an instance of the Builder::class:

Before:

$results = Thread::search('')->where(...)->get();

After:

$results = Thread::search()->where(...)->get();