fix(types): genChildren function with error normalizationType

[skyline0705]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[skyline0705]

bug fix for #9181

first I try to change function genChildren, and change normalizationType to ALWAYS_NORMALIZE, it works in my case

but when I try this code:

var vm = new Vue({
  el: '#demo',
  template: `
      <div>
        <foo v-for="item in list">
          {{ item }}
        </foo>
      </div>
  `,
  data: {
    list: undefined
  },
  components: {
    foo: {
      template: '<div><slot></slot></div>'
    },
  }
})

setTimeout(() => {
  vm.list = [1,2,3,4,5];
}, 1000);

I found it now works because the root element div has no attrs, and found the function createElement has an error params order;

so second I try to change file src/compiler/codegen/index
and change the function call _c, change the code

code = `_c('${el.tag}'${
    data ? `,${data}` : '' // data
  }${
    children ? `,${children}` : '' // children
  })`

to

code = `_c('${el.tag}'${
    data ? `,${data}` : ',undefined' // data
  }${
    children ? `,${children}` : '' // children
  })`

I think it will works, but…… the unit test shows that I'm wrong, there are many error 😭

finally I fix the renderList function, let it always return an array, and everything goes well……

I know use an undefined value in v-for is invalid, but before v2.5.16, it works well, I think it should be warn in the next version like v2.6.x, and should keep work in v2.5.x

[yyx990803]

I think it's better to handle it directly in simpleNormalizeChildren - it avoids allocating empty array on each render and allows the diff algorithm to take a faster path.

See 4748760

[yyx990803]

Ended up using your fix in 847e493 when trying to apply additional optimization for the case... sorry!

[skyline0705]

Ok~ No problem~~And another thing, the fundamental cause of this is because the params order at function createElement, that is why I finally decided to change function renderList: the createElement calls are too flexible😂.

maybe there need a little more test case of compile template, like the test case components with v-for and empty list, root element has no attrs I added before. the steps is on the last comment #9182 (comment)