MOBI6: empty string instead of undefined in TOC

Should fix https://github.com/johnfactotum/foliate/issues/1347

mobi.js

@@ -720,7 +720,7 @@ class MOBI6 {
                     .reduce((arr, a) => {
                         const indent = getIndent(a)
                         const item = {
-                            label: a.innerText?.trim(),
+                            label: a.innerText?.trim() ?? '',
                             href: `filepos:${a.getAttribute('filepos')}`,
                         }
                         const level = indent > lastIndent ? lastLevel + 1