Nil value passed to maskdetails panics

[eleduardo]

Describe the bug
The code handles nil ptrs and other complex stuff well but there is one bug that happens if the value of the instance passed to maskDetails is nil. Of course it is silly to pass nil but sometimes the caller might now know (I hit this on an interceptor on a use case where one object was nil)

The original masker code that this was inspired from does handle that case right away

https://github.com/ggwhite/go-masker/blob/master/masker.go#L111-L113

To Reproduce
Very simple, just

1. Create a new masking instance
2. call masker.MaskDetails(nil)

Expected behavior
This seems like a gray are because it is an error to pass a nil, so either change MaskDetails to pass an error or, if nil is passed, just return nil?