[FIX] purchase_mrp: return string instead or recordset
[FIX] purchase_mrp: return string instead or recordset

Commit e923058 aims to ensure the key 'created_purchase_line_id is
returned only if the move as this field filled. The issue is due to
the way python compare falsy object.
   a = b and c    will return b (not False) if b is falsy
                  will return c if neither b nor c are falsy.

If move_raw_id passed in the method doesn't have created_purchase_line_id
   iterate_key = purchase.line.id() or 'created_purchase_line_id'
               = purchase.line.id()
which is not correct as the method should return a string or False.

closes odoo/odoo#43320

Signed-off-by: Simon Lejeune (sle) <1969d8dff896674c2d819e813f5e88ca826229ab@openerp.com>

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