do not add list key to text nodes (fix #3810)

src/core/vdom/helpers.js

@@ -35,7 +35,7 @@ export function normalizeChildren (
             applyNS(c, ns)
           }
           // default key for nested array children (likely generated by v-for)
-          if (c.key == null && nestedIndex != null) {
+          if (c.tag && c.key == null && nestedIndex != null) {
             c.key = `__vlist_${nestedIndex}_${i}__`
           }
           res.push(c)