[Bug]: Series sequence is not pulled when parsing OPF metadata file

[advplyr]

Describe the issue

When parsing the following stripped down OPF file the series sequence is not pulled because it is not directly under the series name.
This is from the package document in an ebook file.

<?xml version='1.0' encoding='utf-8'?>
<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="BookId" version="2.0">
  <metadata xmlns:calibre="http://calibre.kovidgoyal.net/2009/metadata" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:dcterms="http://purl.org/dc/terms/" xmlns:opf="http://www.idpf.org/2007/opf" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <meta name="calibre:series" content="La Faucheuse"/>
    <meta name="calibre:title_sort" content="FAUCHEUSE 02 Thunderhead"/>
    <meta name="calibre:series_index" content="2.0"/>
  </metadata>
</package>

An adjustment in a recent PR to support multiple series broke this use-case. To support this if there is only 1 series listed then it should use the series_index no matter where it is.

Steps to reproduce the issue

1. Create OPF file with XML above and place inside a new folder with an ebook
2. Scan in the new book
3. Observe a series with no sequence

Audiobookshelf version

v2.7.1

How are you running audiobookshelf?

Built from source

[advplyr]

Fixed in https://github.com/advplyr/audiobookshelf/releases/tag/v2.7.2