Disable the {filter: {q: ""}} created by default from ReferenceInput

[multimeric]

Is your feature request related to a problem? Please describe.
By default, a <ReferenceInput> component generates a GET_LIST request, with the params set to {filter: {q: ""}}, aka an empty filter for the column q.

This is a strange behaviour, because backends may actually try to interpret this filter, even though it has no value. For example, flask-rest-jsonapi tries to filter by a column called q, which doesn't exist. Similarly, discussion in #2615 shows this problem in a different backend.

Describe the solution you'd like
By default, a <ReferenceInput> should not add any parameters to the GET_LIST request. Instead, users should be required to implement a filterToQuery function, where they can add their own filter if needed.

In terms of the actual code, I think the only thing we'd need to do is change the default prop value of filterToQuery to a no-op:

react-admin/packages/ra-ui-materialui/src/input/ReferenceInput.tsx

Line 162 in 0b08e99

filterToQuery: searchText => ({ q: searchText }),

Describe alternatives you've considered
Currently this default behaviour can be disabled by adding a no-op filterToQuery, e.g.:

<ReferenceInput
    filterToQuery={() => {}}
>

I'd be happy to write a PR if this would be acceptable.

[fzaninotto]

Thanks, reproduced and confirmed. I'll post a fix.