Fix RowClickFunction type to accept false.

[stesie]

DatagridRow's rowClick property may be provided with a function, that may return a string (to trigger navigation) or false (in order to skip navigation). At least the code would allow to do that, see https://github.com/marmelab/react-admin/blob/master/packages/ra-ui-materialui/src/list/datagrid/DatagridRow.tsx#L113

However typing restricts this (via RowClickFunction) to just support string return types (or promises there of).

This PR changes the type to allow false to be returned as well (both directly or with promise).

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Please note that there are further inconsistencies with typing of rowClick on Datagrid itself and DatagridBody, which both disallow rowClick={false} (opposed to DatagridRow). Furthermore DatagridRow.propTypes.rowClick does not allow false value (i.e. conflicts with typing).
This PR ignores this so far, since I'm not directly affected by that and I wasn't sure whether to fix that. Let me know if I should.

[djhi]

Thanks!