[BO] Bugfix: read transaction_id from array

empty transaction id error occurs if someone cancels a product. Code tries to read id from object but an array is given. See http://www.prestashop.com/forums/topic/234174-how-to-solve-fatal-error-id-transaction-is-null-when-refundingcancelling-orders/ as well...
Merge pull request #172 from Shoprunners/patch-1

PayPal : Bug fix - Read transaction_id from array