Make Table::get_search_index not throw
Make Table::get_search_index not throw

If search index is not present, just return nullptr.

Change-Id: Iaf41d1fbab744b91c77c70deca310cae525671af

realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core