handling null value in nested entity of solr data importer - solr

Question

I'm using the Solr Data Importer to import some category data. I didn't want to use a left join in the parent query because it's too complicated, I preferred to use nested object queries in the configuration to keep it simple.
I've got 3 one to one relationships for feature images of a category. My question is though, how can I handle it when the value in mediaItemX_id field is null? I've tried the nested configuration below, but when the value is null it's reporting invalid sql because the nested query doesn't print null - it prints blank....
<entity name="category" query="SELECT concat('CATEGORY_', c.id) as docId, c.id, externalIdentifier, name, description, shortDescription, mediaItem1_id, mediaItem2_id, mediaItem3_id, created, lastUpdated, keywords, 'CATEGORY' as docType,
name as autoSuggestField
FROM categories c inner join base_content bc where c.id = bc.id">
<field column="id" name="categoryId" />
<field column="externalIdentifier" name="externalIdentifier" />
<field column="docType" name="docType" />
<field column="name" name="name" />
<field column="description" name="description" />
<field column="shortDescription" name="shortDescription" />
<field column="created" name="created" dateTimeFormat="yyyy-MM-dd'T'HH:mm:ss" />
<field column="lastUpdated" name="lastUpdated" dateTimeFormat="yyyy-MM-dd'T'HH:mm:ss" />
<field column="publishDate" name="publishDate" dateTimeFormat="yyyy-MM-dd'T'HH:mm:ss" />
<field column="archiveDate" name="archiveDate" dateTimeFormat="yyyy-MM-dd'T'HH:mm:ss" />
<field column="autoSuggestField" name="suburbSuggest" />
<field column="keywords" name="keywords" />
<entity name="mediaItem1" query="SELECT uri, title, altText from media where ${category.mediaItem1_id} is not null and id = ${category.mediaItem1_id}">
<field column="uri" name="featureImage1Url" />
<field column="title" name="featureImage1Title" />
<field column="altText" name="featureImage1AltText" />
</entity>
<entity name="mediaItem2" query="SELECT uri, title, altText from media where ${category.mediaItem2_id} is not null and id = ${category.mediaItem2_id}">
<field column="uri" name="featureImage2Url" />
<field column="title" name="featureImage2Title" />
<field column="altText" name="featureImage2AltText" />
</entity>
<entity name="mediaItem1" query="SELECT uri, title, altText from media where ${category.mediaItem3_id} is not null and id = ${category.mediaItem3_id}">
<field column="uri" name="featureImage3Url" />
<field column="title" name="featureImage3Title" />
<field column="altText" name="featureImage3AltText" />
</entity>
</entity>

Answer 1

Solr supports the notion ${value:default} as replacements in other locations, so I'd try that at least:
${category.mediaItem1_id} IS NOT NULL AND id = ${category.mediaItem1_id:0}
I were unable to find a decent way to skip the entities whole if the current value is false.