tarantool_graphql::nullable for nesting NonNulls

It is for make the function general and don't have fears of corner
cases. It will loop in case of x = { ofType = x }, but it is anyway
incorrect input.

Author: Totktonada

graphql/tarantool_graphql.lua

@@ -58,15 +58,14 @@ local function avro_type(avro_schema)
     error('unrecognized avro-schema type: ' .. json.encode(avro_schema))
 end
 
--- XXX: recursive skip several NonNull's?
 local function nullable(gql_class)
     assert(type(gql_class) == 'table', 'gql_class must be a table, got ' ..
         type(gql_class))
 
     if gql_class.__type ~= 'NonNull' then return gql_class end
 
     assert(gql_class.ofType ~= nil, 'gql_class.ofType must not be nil')
-    return gql_class.ofType
+    return nullable(gql_class.ofType)
 end
 
 local types_long = types.scalar({