Incorrect cell validation / coercion

[Marc-Andre-Rivet]

1 - Validation default is not applied correctly when its value is 0 (number) -- the value is falsy and trips the default case
2 - Deleting cell content with backspace does not run validation

1 - This is simple, update https://github.com/plotly/dash-table/blob/dev/src/dash-table/type/reconcile.ts#L67 to do a R.isNil check instead

2 - This is a bit more involved, I suggest that we start using the on_change settings applicable to each cell and use the reconciliation result of null if the reconciliation is successful. Otherwise, continue using '' as we do right now.