Allow superDecoder to wrap null values

* Update {JSON,PropertyList}Decoder to allow superDecoder()s to wrap
  null value so current implementations of collections can decode
  contained objects from nil

Author: Itai Ferber

stdlib/public/SDK/Foundation/JSONEncoder.swift

@@ -1281,12 +1281,7 @@ fileprivate struct _JSONKeyedDecodingContainer<K : CodingKey> : KeyedDecodingCon
 
     private func _superDecoder(forKey key: CodingKey) throws -> Decoder {
         return try self.decoder.with(pushedKey: key) {
-            guard let value = self.container[key.stringValue] else {
-                throw DecodingError.keyNotFound(key,
-                                                DecodingError.Context(codingPath: self.codingPath,
-                                                                      debugDescription: "Cannot get superDecoder() -- no value found for key \"\(key.stringValue)\""))
-            }
-
+            let value = self.container[key.stringValue]
             return _JSONDecoder(referencing: value, at: self.decoder.codingPath, options: self.decoder.options)
         }
     }
@@ -1631,12 +1626,6 @@ fileprivate struct _JSONUnkeyedDecodingContainer : UnkeyedDecodingContainer {
             }
 
             let value = self.container[self.currentIndex]
-            guard !(value is NSNull) else {
-                throw DecodingError.valueNotFound(Decoder.self,
-                                                  DecodingError.Context(codingPath: self.codingPath,
-                                                                        debugDescription: "Cannot get superDecoder() -- found null value instead."))
-            }
-
             self.currentIndex += 1
             return _JSONDecoder(referencing: value, at: self.decoder.codingPath, options: self.decoder.options)
         }

stdlib/public/SDK/Foundation/PlistEncoder.swift

@@ -1053,12 +1053,7 @@ fileprivate struct _PlistKeyedDecodingContainer<K : CodingKey> : KeyedDecodingCo
 
     private func _superDecoder(forKey key: CodingKey) throws -> Decoder {
         return try self.decoder.with(pushedKey: key) {
-            guard let value = self.container[key.stringValue] else {
-                throw DecodingError.valueNotFound(Decoder.self,
-                                                  DecodingError.Context(codingPath: self.codingPath,
-                                                          debugDescription: "Cannot get superDecoder() -- no value found for key \"\(key.stringValue)\""))
-            }
-
+            let value = self.container[key.stringValue]
             return _PlistDecoder(referencing: value, at: self.decoder.codingPath, options: self.decoder.options)
         }
     }
@@ -1402,12 +1397,6 @@ fileprivate struct _PlistUnkeyedDecodingContainer : UnkeyedDecodingContainer {
             }
 
             let value = self.container[self.currentIndex]
-            guard !(value is NSNull) else {
-                throw DecodingError.valueNotFound(Decoder.self,
-                                                  DecodingError.Context(codingPath: self.codingPath,
-                                                          debugDescription: "Cannot get superDecoder() -- found null value instead."))
-            }
-
             self.currentIndex += 1
             return _PlistDecoder(referencing: value, at: self.decoder.codingPath, options: self.decoder.options)
         }