Fix crash in more complex content around attention

Related-to: 97fb818.
Closes GH-63.

Author: wooorm

lib/util/container-phrasing.js

@@ -102,6 +102,7 @@ export function containerPhrasing(parent, state, info) {
     // encode it.
     if (encodingInfo) {
       if (
+        results.length > 0 &&
         encodingInfo.before &&
         before === results[results.length - 1].slice(-1)
       ) {