Show header row for empty Datagrids

[JoeJErnst]

Is your feature request related to a problem? Please describe.
I don't understand why the decision was made to return null for the whole Datagrid if there are no rows to return. I would expect to still see the header row.

Describe the solution you'd like
Offer a property that allows me to specify whether or not to hide empty Datagrids.

Describe alternatives you've considered
It could be a boolean hideWhenEmpty (default to true for backwards compatibility).

Additional context

react-admin/packages/ra-ui-materialui/src/list/Datagrid.js

Line 176 in 5b44d97

if (loaded && (ids.length === 0 || total === 0)) {

[fzaninotto]

The <List> component exposes an empty prop exactly for this use case.

https://marmelab.com/react-admin/List.html#empty-empty-page-component

[JoeJErnst]

The empty prop on the <List> component is far too restrictive for this use case, for three separate reasons:

1. It only works if there's NOT an active filter
2. It only works if there's a create page for the resource
3. It still won't show the column headers

I have several Datagrids in my app that will have default filters enabled, and no create page. I still need my users to see that there's an empty grid, not no grid at all. There is value in seeing what the columns are, even if no data exists.

I feel this should be true in general, not just because my app has special requirements. I stand by my enhancement request for the Datagrid component. I'll see if I can get a pull request put together with my suggested implementation, in case you want to reconsider.

[fzaninotto]

I disagree that showing a header line with no body is better in general, but I understand that it may be better in some cases.

About your points:

1. Right, the empty page is designed as the initial state, not for displaying an empty filtered list
2. no, the <List empty> component displays even though there is no create page
3. it's up to you to pass the component of your choice to display just a list of headers in this case

Also, the <Datagrid> could accept an empty prop to display a custom component instead of null when the list is empty because the filters are too narrow. We can accept a PR for that (and I think it's trivial).