fix: 🩹 allow no affiliations in crossref metadata

scipost_django/journals/templates/xml/_person_name_crossref.html

 <person_name sequence="{% if publication_author.order == 1 %}first{% else %}additional{% endif %}" contributor_role='author'>
   <given_name>{{ publication_author.first_name }}</given_name>
   <surname>{{ publication_author.last_name }}</surname>
-  <affiliations>
-    {% for affiliation in publication_author.affiliations.all %}
-      <institution>
-        <institution_name>{{ affiliation.name }}</institution_name>
-        {% if affiliation.ror_json.ror_link %}
-          <institution_id type="ror">{{ affiliation.ror_json.ror_link }}</institution_id>
-        {% else %} {% comment %} Only if ror doesn't exist, acronyms may help {% endcomment %}
-          {% if affiliation.acronym %}
-            <institution_acronym>{{ affiliation.acronym }}</institution_acronym>
+  {% if publication_author.affiliations.all %}
+    <affiliations>
+      {% for affiliation in publication_author.affiliations.all %}
+        <institution>
+          <institution_name>{{ affiliation.name }}</institution_name>
+          {% if affiliation.ror_json.ror_link %}
+            <institution_id type="ror">{{ affiliation.ror_json.ror_link }}</institution_id>
+          {% else %} {% comment %} Only if ror doesn't exist, acronyms may help {% endcomment %}
+            {% if affiliation.acronym %}
+              <institution_acronym>{{ affiliation.acronym }}</institution_acronym>
+            {% endif %}
           {% endif %}
-        {% endif %}
-      </institution>
-    {% endfor %}
-  </affiliations>
+        </institution>
+      {% endfor %}
+    </affiliations>
+  {% endif %}
   {% if publication_author.profile.orcid_id %}
     <ORCID authenticated="{{ publication_author.profile.orcid_authenticated|yesno:"true,false" }}">https://orcid.org/{{ publication_author.profile.orcid_id }}</ORCID>
   {% endif %}