fix: for throws undefined var with a null value with strictVariables

src/context/context.ts

@@ -2,7 +2,7 @@ import { Drop } from '../drop/drop'
 import { __assign } from 'tslib'
 import { NormalizedFullOptions, defaultOptions, RenderOptions } from '../liquid-options'
 import { Scope } from './scope'
-import { isArray, isNil, isString, isFunction, toLiquid, InternalUndefinedVariableError, toValueSync } from '../util'
+import { isArray, isNil, isUndefined, isString, isFunction, toLiquid, InternalUndefinedVariableError, toValueSync } from '../util'
 
 type PropertyKey = string | number;
 
@@ -80,7 +80,7 @@ export class Context {
     if (isString(paths)) paths = paths.split('.')
     for (let i = 0; i < paths.length; i++) {
       scope = yield readProperty(scope as object, paths[i], this.ownPropertyOnly)
-      if (isNil(scope) && this.strictVariables) {
+      if (this.strictVariables && isUndefined(scope)) {
         throw new InternalUndefinedVariableError((paths as string[]).slice(0, i + 1).join!('.'))
       }
     }

src/util/underscore.ts

@@ -63,6 +63,10 @@ export function isNil (value: any): boolean {
   return value == null
 }
 
+export function isUndefined (value: any): boolean {
+  return value === undefined
+}
+
 export function isArray (value: any): value is any[] {
   // be compatible with IE 8
   return toString.call(value) === '[object Array]'

test/integration/tags/for.spec.ts

@@ -59,6 +59,12 @@ describe('tags/for', function () {
     const html = await liquid.parseAndRender(src, scope)
     return expect(html).toBe('str-"string"-string')
   })
+  it('should not report undefined variable on null value', async function () {
+    const engine = new Liquid({ strictVariables: true })
+    const src = '{% assign hello = "hello,world" | split: "," | concat: null %}{% for i in hello %}{{ i }},{% endfor %}'
+    const html = await engine.parseAndRender(src, scope)
+    return expect(html).toBe('hello,world,,')
+  })
   describe('illegal', function () {
     it('should reject when for not closed', function () {
       const src = '{%for c in alpha%}{{c}}'