Easy access TermStates's needStats flag

### Description

When we build TermStates we pass a flag needStats to determine if we want to front loading all term statistics, however, we did not have easy access to know if this flag has been set to true or not. This will just be a simple PR that add an api to check if the term is not null.

Start by locating TermStates and inspect how its needStats value is stored or used. Add the requested API for checking whether statistics were requested, then verify that the result reflects the flag for both enabled and disabled cases.

Written by the indexing model from the issue text.