Check the corner case in an avro-schema generation

This follows up the PR #58 (the work in the scope of the issue #7).

Author: Totktonada

graphql/query_to_avro.lua

@@ -4,6 +4,8 @@
 ---
 --- * The best way to use this module is to just call `avro_schema` method on
 ---   compiled query object.
+
+local json = require('json')
 local path = "graphql.core"
 local introspection = require(path .. '.introspection')
 local query_util = require(path .. '.query_util')
@@ -41,20 +43,38 @@ end
 --- In the current tarantool/avro-schema implementation we simply add '*' to
 --- the end of a type name.
 ---
---- If the type is already nullable the function leaves it as is.
+--- If the type is already nullable the function leaves it as if
+--- `opts.raise_on_nullable` is false or omitted. If `opts.raise_on_nullable`
+--- is true the function will raise an error.
 ---
 --- @tparam table avro avro schema node to be converted to nullable one
 ---
+--- @tparam[opt] table opts the following options:
+---
+--- * `raise_on_nullable` (boolean) raise an error on nullable type
+---
 --- @result `result` (string or table) nullable avro type
-local function make_avro_type_nullable(avro)
+local function make_avro_type_nullable(avro, opts)
     assert(avro ~= nil, "avro must not be nil")
+    local opts = opts or {}
+    assert(type(opts) == 'table',
+        'opts must be nil or a table, got ' .. type(opts))
+    local raise_on_nullable = opts.raise_on_nullable or false
+    assert(type(raise_on_nullable) == 'boolean',
+        'opts.raise_on_nullable must be nil or a boolean, got ' ..
+        type(raise_on_nullable))
 
     local value_type = type(avro)
 
     if value_type == "string" then
-        return avro:endswith("*") and avro or (avro .. '*')
+        local is_nullable = avro:endswith("*")
+        if raise_on_nullable and is_nullable then
+            error('expected non-null type, got the nullable one: ' ..
+                json.encode(avro))
+        end
+        return is_nullable and avro or (avro .. '*')
     elseif value_type == "table" then
-        return make_avro_type_nullable(avro.type)
+        return make_avro_type_nullable(avro.type, opts)
     end
 
     error("avro should be a string or a table, got " .. value_type)
@@ -103,7 +123,7 @@ local function gql_type_to_avro(fieldType, subSelections, context)
     end
 
     if not isNonNull then
-        result = make_avro_type_nullable(result)
+        result = make_avro_type_nullable(result, {raise_on_nullable = true})
     end
     return result
 end