12492: fix rendering to XML if there's a disabled cascading select questioning

[cooperka]

scenario:

• form has a cascading (multilevel) select that's disabled
• DelayedJob tries to render the form to XML automatically

result:

• undefined method 'body_tags' for nil:NilClass

fix:

• don't attempt to render non-existent child

note that BOTH the if and else eventually attempt to use visible_children[0], but they only currently check if there are children and not whether the children are visible. this code logic is extremely complex and branching, but the fix here is likely safe.

tested and verified using the prod form linked from the redmine issue (screenshots not uploaded for privacy reasons).

[jpdimond]

Looks good to me kevin, thank you!