I don't know if there is an easy way to address the key crux of your
question...
: only applied of there are 0 values for the field. Is there a way when
: using the DIH to replace 'null' or missing values with a default, such
: that I can ensure that I always have the same number of values in each
: multivalued field?
...but one other idea i'd like to put out there for you to consider is to
change how you think about your index, so that instead of having one
document per "root" (in your sample data) you create one document per
"style" -- this would let you query on individual styles very easily, and
you could use Grouping when you need to find "root" products...
https://wiki.apache.org/solr/FieldCollapsing
...I don't know enough about your usecase to know if this approach would
make your life easier or more complicated, but it's something to consider.
-Hoss
