[FIX] fields property: do not create empty properties

fixes #595
In the case where a property for the company exists but has no related record (e.g. in case of type m2o with no defined value), not setting a value to this field for a new record would create a new property (as browse_null is not an instance of browse_record)
[FIX] purchase_requisition: duplicated stock moves

When a purchase requisition is created from a procurement order, a first stock move is created, not associated to any purchase orders
Then, on purchase order creation and confirmation, in the purchase requisition, new stock moves are created, associated to the purchase order.
The existing stock move issued from the procurement order which created the purchase requisition remained untouched, leading to wrong inventory values
To fix this, the destination location of the stock move of the procurement order is written on the source location
A proper fix should be to use a dedicated workflow for puchase requisition, but this can't be done in 7.0, it has to be done in master/trunk
[FIX] account: add multicompany rule to account.subscription.line

The wizard to generate recurring entries did not respect multicompany rules as the request was done in plain sql.
Add ir.rule and use the orm (opw 607782)