Propagate options to 1-child -reducing, disallow 0-child

[frenchy64]

:union and :merge don't work well with 0 and 1 children.

For 1 child, they don't propagate options to the child. This happens because the reduce call in -reducing returns init, which is a ?schema.

• this pr ensures options are always propagated

For 0 children, deref'ing returns nil. This is because the reduce call in -reducing is passed nil init.

• this pr adds checks for non-empty children to :merge and :union
• fallback error for other users of -reducing

(m/form (m/deref-all (m/schema [:merge [:merge :map]] {:registry (merge (mu/schemas) (m/default-schemas))})))
; (err) Execution error (ExceptionInfo) at malli.core/-exception (core.cljc:157).
; (err) :malli.core/invalid-schema

(m/deref (m/schema :merge {:registry (merge (mu/schemas) (m/default-schemas))}))
;=> nil

[ikitommi]

Thanks!