page_image is currently always set and added to the page_props table, even if there is no relevant image. Instead of not adding the property, the property is added with a blank value.
e.g. for https://www.wikidata.org/wiki/Q5160
select * from page_props where pp_page = 6102;
+---------+--------------+------------------------------------------------------------------------------------------------------------------------------------------+------------+
| pp_page | pp_propname | pp_value | pp_sortkey |
+---------+--------------+------------------------------------------------------------------------------------------------------------------------------------------+------------+
| 6102 | displaytitle | <span class="wikibase-title ">
<span class="wikibase-title-label">'Are'are</span>
<span class="wikibase-title-id">(Q5160)</span>
</span> | NULL |
| 6102 | page_image | | NULL |
| 6102 | wb-claims | 7 | 7 |
| 6102 | wb-sitelinks | 6 | 6 |
+---------+--------------+------------------------------------------------------------------------------------------------------------------------------------------+------------+
4 rows in set (0.00 sec)
we should instead should just not set the page_image property in the ParserOutput in this case. This is how it is done for wikitext pages.