fix validation for non-nullable arguments

The problem was introduced in e37651d
(support "null" arguments) here we add redundant check that
started to throw error "graphql/util.lua:72: attempt to index local 'node' (a nil value)"
This patch removes this check and adds test (with small difference
copied from TDG) that catches a problem.

Author: olegrok

graphql/util.lua

@@ -68,22 +68,18 @@ local function coerceValue(node, schemaType, variables, opts)
   local strict_non_null = opts.strict_non_null or false
   local defaultValues = opts.defaultValues or {}
 
-  if not node then
-    return nil
-  end
-
   if schemaType.__type == 'NonNull' then
-    if node.kind == 'null' then
-      error(('Expected non-null for "%s", got null'):format(getTypeName(schemaType)))
-    end
-
     local res = coerceValue(node, schemaType.ofType, variables, opts)
     if strict_non_null and res == nil then
       error(('Expected non-null for "%s", got null'):format(getTypeName(schemaType)))
     end
     return res
   end
 
+  if not node then
+    return nil
+  end
+
   -- handle precompiled values
   if node.compiled ~= nil then
     return node.compiled

test/integration/graphql_test.lua

@@ -1086,3 +1086,37 @@ function g.test_null()
             end
     )
 end
+
+function g.test_validation_non_null_argument_error()
+     local query = [[
+        query QueryFail {
+            TestEntity(insert: {})
+        }
+    ]]
+
+    local function callback(_, _)
+        return nil
+    end
+
+    local query_schema = {
+        ['TestEntity'] = {
+            kind = types.string,
+            arguments = {
+                insert = types.inputObject({
+                    name = 'TestEntityInput',
+                    fields = {
+                        non_null = types.string.nonNull,
+                    }
+                }),
+            },
+            resolve = callback,
+        }
+    }
+
+    t.assert_error_msg_contains(
+            'Expected non-null',
+            function()
+                check_request(query, query_schema, {variables = {}})
+            end
+    )
+end