Update PlistEncoder with proposed nil changes
Update PlistEncoder with proposed nil changes

* Add encodeNil/decodeNil variants on keyed and unkeyed containers
* Give implementations of decode() variants instead of decodeIfPresent()

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation