onRowSelectionChange disables SelectToolbar

When implementing `onRowSelectionChange` (which according to docs should return `void`) 3 things happen:
- select works for just one row, and only for select (and not deselect)
- select button doesn't change (no V state)
- `SelectToolbar` doesn't appear (no matter the `selectToolbarPlacement` value)

## Expected Behavior

`onRowSelectionChange` should only be a listener to selection function, and I suspect it's deeper (haven't explored the internals).
That way, we can encapsulate the selection change with different implementations (basic example: state controlling whether or not there are any selected rows), without affecting the selection behavior itself.

## Current Behavior

Seems like `onRowSelectionChange` either controls the selection behavior and we override it, or it should return something that isn't documented.

## Steps to Reproduce (for bugs)

Just implement `onRowSelectionChange` and you'll see.
Edited the official example for `CustomToolbar` to showcase the behavior:
https://codesandbox.io/s/festive-chebyshev-til1i?file=/examples/customize-toolbar/index.js

## Your Environment
Used the official Examples CodeSandbox, so I'm assuming those are the most relevant versions.

Reproduce the issue using the official CustomToolbar example at examples/customize-toolbar/index.js and the linked CodeSandbox, with onRowSelectionChange enabled. Trace how that callback affects row selection and SelectToolbar rendering, then verify that selection, deselection, the selected-state indicator, and SelectToolbar still work while the callback remains a listener.

Written by the indexing model from the issue text.