Support rules for undefined partial documents not generated correctly

[tsandall]

PE generates support rules for partial documents if the reference requires the full extent of the document. However, if none of the rules that generate the document are defined, the document should be considered empty (not undefined). Currently PE does not generate the correct output in this case (it generates a reference to a non-existent/undefined virtual document.)

Example:

data.foo.p = x

package foo

p[1] { input; false }

In this case, when PE runs on the query above, the body of p is undefined (because of the expression "false") however the current output is an empty set (undefined). We can fix this by unifying the other term in the query with the empty value for the virtual document (i.e., set() or {}).