backend/hafas: make productName optional

Some legs do not specify a productName.

src/backend/hafas/types.nim

@@ -29,8 +29,8 @@ type
   HafasProdCtx* = object
     name*:     string
     num*:      string
-    catOut*:   string
-    catOutL*:  string
+    catOut*:   Option[string]
+    catOutL*:  Option[string]
     lineId*:   Option[string]
 
   HafasProd* = object

src/types.nim

@@ -83,8 +83,8 @@ type
     mode*:              Mode
     product*:           Product
     tripNum*:           string
-    productName*:       string
-    fullProductName*:   string
+    productName*:       Option[string]
+    fullProductName*:   Option[string]
     operator*:          Option[Operator]
     additionalName*:    Option[string]