recoverWithNull() and replaceWithNull()

[gavinking]

Due to overloading, UniOnFailure.recoverWithItem(null) and Uni.replaceWith(null) result in compilation errors, forcing me to write either recoverWithItem((Void) null) or recoverWithItem(()->null).

I would like to request new methods to make it easy to obtain a Uni<Void>. If recoverWithNull() and replaceWithNull() seem too verbose, they could be named recover() and discard().

[cescoffier]

I like recoverWithNull() and replaceWithNull(), the names convey what it does.

[jponge]

I will do

[jponge]

See #324