query: Report data type for unary operators

All data kinds (used to restrict the data collected) referenced in a
filter can be requested once it's been “compiled”. However, the kinds
of fields used in boolean expressions (e.g. ["?", "xyz"]) were not
recorded. This patch changes the code accordingly and provides a unit
test update.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>