Add mapNullable function to Stores and Inspectors

[haukesomm]

This PR adds a mapNullable function to Stores and Inspectors that works as the reverse equivalent for mapNull.

The function can be used to map non-nullable Stores/Inspectors to nullable ones as follows:

val nonNullableStore: Store<String> = storeOf("")

val nullableStore: Store<String?> = 
    nonNullableStore.mapNullable(placeholder = "Unknown")
//                               ^^^^^^^^^^^^^^^^^^^^^^^
//                               When the parent has the specified placeholder value,
//                               the mapped Store will have `null` as its value.

Closes #927

[Lysander]

Please check the remarked question first.

[Lysander]

Sure that this is a good idea? If the value of a Lens's id is an empty String, we do not modify the path like for formatting lenses.

How was this function used before?

Does this maybe affect the validation process for generated lenses? Have you checked this?

[haukesomm]

Good point! The parameter has never been used, though. I manually checked every occurrence and made sure it's an empty String. As the function is internal, we can be sure that it is not breaking as well 👍🏻

[Lysander]

I think the new function is also missing in the docs:
https://www.fritz2.dev/docs/storemapping/#summary-of-store-mapping-factories