Nested map with invalid value breaks `strip-extra-keys-transformer` when using `map-of`

[bgalartza]

Apparently having a nested map with an invalid value inside map-of breaks the strip-extra-keys-transformer.

Works as expected:

(m/encode
 [:map-of
  :any
  [:map
   [:a
    [:string]]
   [:b
    [:string]]]]
 {"anything" {:a "a" :b "b" :c "c"}}
 mt/strip-extra-keys-transformer)
{"anything" {:a "a", :b "b"}

Produces empty map instead of {"anything" {:a "a"}}:

(m/encode
 [:map-of
  :any
  [:map
   [:a
    [:string]]
   [:b
    [:string]]]]
 {"anything" {:a "a" :c "c"}}
 mt/strip-extra-keys-transformer)
 {}

I found some mention to this problem in this pull request

[opqdonut]

might be related to #1112

[ikitommi]

This is the current behavior. :map-of validates it's child is valid and strips if not. I think a better behavior would be here to check if the key & value are of just correct type. Currently, there is no such mechanism (just type checking) in malli. Should be, IMO.