fix exception on "template" elements

[vaab]

This patch is over 2.4.0 (I tried 4.0.0-alpha-8 with too many broken things).

It fixes:

        element.content.childNodes.forEach(function (childNode) {
                        ^

TypeError: Cannot read properties of undefined (reading 'childNodes')
    at getElementContent (/home/vaab/dev/js/monujo/node_modules/vue3-gettext/dist/bin/gettext_extract.js:93:25)
    at /home/vaab/dev/js/monujo/node_modules/vue3-gettext/dist/bin/gettext_extract.js:112:26
    at HtmlParser.parseNode (/home/vaab/dev/js/monujo/node_modules/gettext-extractor/dist/html/parser.js:19:13)

When encountering this file test.vue with this content:

<template>
  <svg>
    <template v-for="x in [1, 2]">
    </template>
  </svg>
</template>

I'm aware that this is a quick workaround rather than a wise thoughtful patch.