topdown: is_valid functions no longer error. (#4844)

This commit updates the `*.is_valid` functions to no longer produce
errors when providing wrong-typed arguments. Instead, they will now
return true/false for all inputs. Tests and WASM versions of these
builtins have been updated to match the new behavior.

Fixes #4760.

Signed-off-by: Philip Conrad <philipaconrad@gmail.com>