fix(transition-group): handle non-visible list items, fix #8516

[yannisgu]

Handle the case, when a component inside a transition-group doesn't yield any HTML, e.g. when there is a v-if on the root-element.

Before this fix, transition-group would fail with the errror c$1.elm.getBoundingClientRect is not a function, when moving items, if there was such an item.

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:

[DabeDotCom]

FWIW, I just ran into a similar problem (even under 2.6.12) and fixed worked around it by adding:

--- a/transition-group.js	2021-03-04 14:24:20.000000000 -0500
+++ b/transition-group.js	2021-03-04 14:25:18.000000000 -0500
@@ -79,6 +79,7 @@ export default {
       const removed: Array<VNode> = []
       for (let i = 0; i < prevChildren.length; i++) {
         const c: VNode = prevChildren[i]
+        if (!c) continue
         c.data.transition = transitionData
         c.data.pos = getBoundingClientRect(c)
         if (map[c.key]) {