chore: improve map cast functions

[sagikazarmark]

No description provided.

[andig]

@sagikazarmark the downside with this change is that any function ignoring the the error (i.e. without E suffix) will now receive a nil instead of an empty map. That's unexpected compared to older versions and doesn't quite fit the slogan of

safe and easy casting from one type to another in Go

I couldn't determine from the code if you strictly require this change for anything? If not I would propose to always return the empty map instead of nil, as before, as a sane and safe default.