[DataGrid] apiRef.current.getRow can return null

[flaviendelangle]

See #1995 (comment)

For getRowParams / getCellParams / getBaseCellParams I could return null instead of throwing, but it would mean checking if the return value is null in a lot more places.

For getCellValue if I return null we could not differentiate a cell with the value null and a cell with a non existing ID

For selectRow, I had to update the test because we were asking for a row that do not exist but we never checked the return value of the function in the tests. I don't know what is the best behavior here, does it make sense to return null instead of crashing ?