Add handling of empty Many to Many field on insert

Found an issue where a many to many column with `allowNull=true` will return an error on an `/table/add` but not a `/table/:id`

This happens because the column value on a `/table/add` will be `undefined` when it reaches the template.validate() method here: https://github.com/simov/express-admin/blob/master/lib/utils/template.js#L69
Whereas on an update the same column value will be set to `[]` which is explicitly checked for in template.validate()

This discrepancy happens from a check in editview.getIds() that exits before setting the `[]` value in the case of no primary key being set ( which is the case during an insert )