JATS: Improve decoding of keywords

[nokome]

This JATS XML contains the following:

<kwd-group kwd-group-type="author-keywords">
  <kwd>open access</kwd>
  <kwd>meta-research</kwd>
  <kwd>research policy</kwd>
  <kwd>universities</kwd>
  <kwd>scholarly publishing</kwd>
  <kwd>repositories</kwd>
</kwd-group>
<kwd-group kwd-group-type="research-organism">
  <title>Research organism</title>
  <kwd>None</kwd>
</kwd-group>

which is decoded to

  "keywords": [
   "open access",
   "meta-research",
   "research policy",
   "universities",
   "scholarly publishing",
   "repositories",
   "None"
  ],

The "None" keyword should not be included.