[fix] ngrams_url can be optional as it turns out

src/OpenAlex/Types.hs

@@ -261,7 +261,7 @@ data Work = Work
   , locations                     :: [Location]
   , locations_count               :: Count
   , mesh                          :: [MeSH]
-  , ngrams_url                    :: URL
+  , ngrams_url                    :: Maybe URL
   , open_access                   :: OpenAccess
   , primary_location              :: Maybe Location
   , publication_date              :: CreatedDate
@@ -303,7 +303,7 @@ instance FromJSON Work where
     locations <- v .: "locations"
     locations_count <- v .: "locations_count"
     mesh <- v .: "mesh"
-    ngrams_url <- v .: "ngrams_url"
+    ngrams_url <- v .:? "ngrams_url"
     open_access <- v .: "open_access"
     primary_location <- v .:? "primary_location"
     publication_date <- v .: "publication_date"