[commodity-utils] partially revert eac79bd

The eac79bd commit was slightly overzealous - if pair was #f,
originally this make-exchange-function would create a function that
returns (gnc:make-gnc-monetary domestic 0); I thought it was
acceptable to return #f instead but this would lead to report
crash. Revert previous behaviour.

gnucash/report/report-system/commodity-utilities.scm

@@ -698,10 +698,11 @@ construct with gnc:make-gnc-monetary and gnc:monetary->string instead.")
              (gnc:exchange-if-same foreign domestic)
              (let* ((foreign-comm (gnc:gnc-monetary-commodity foreign))
                     (pair (assoc foreign-comm exchange-alist)))
-               (and pair
-                    (gnc:make-gnc-monetary
-                     domestic
-                     (* (gnc:gnc-monetary-amount foreign) (cadr pair)))))))))
+               (gnc:make-gnc-monetary
+                domestic
+                (if pair
+                    (* (gnc:gnc-monetary-amount foreign) (cadr pair))
+                    0)))))))
 
 ;; Helper for the gnc:exchange-by-pricalist* below. Exchange the
 ;; <gnc:monetary> 'foreign' into the <gnc:commodity*> 'domestic' by