Pre-Condtion in `sqb/compile-schema` suggests only `columns` and `idents` is needed, but fails later for several other keys about walkable

[myguidingstar]

Great discovery! I think the above three should not be mandatory.
flatten-multi-keys should only be applied after checking for non nil. Eg:

(flatten-multi-keys extra-conditions)

should become

(when extra-conditions
  (flatten-multi-keys extra-conditions))

are you willing to make another pr? :)

[christoph-frick]

What about putting them in the :or as empty maps?

walkable/src/walkable/sql_query_builder.cljc

Line 419 in 9621f1c

:or {quote-marks backticks}

[myguidingstar]

that should work, too. Maybe that would save us from some null point exception headaches that I can't contemplate atm