Add optional value to ExplicitNull and ServerTimestamp (#3784)

* Add optional value to ExplicitNull and ServerTimestamp

* Fixing comments

* another comment fix

Author: wu-hui

Firestore/Swift/Source/Codable/ExplicitNull.swift

@@ -38,8 +38,8 @@ public enum ExplicitNull<Wrapped> {
     }
   }
 
-  /// Get the `Optional` representation of `ExplicitNull`.
-  public var value: Wrapped? {
+  /// Returns this value as an `Optional<Wrapped>`.
+  public var optionalValue: Wrapped? {
     switch self {
     case .none:
       return .none

Firestore/Swift/Source/Codable/FieldValue+Encodable.swift

@@ -49,6 +49,19 @@ public enum ServerTimestamp: Codable, Equatable {
   /// value to `stamp`.
   case resolved(Timestamp)
 
+  /// Returns this value as an `Optional<Timestamp>`.
+  ///
+  /// If the server timestamp is still pending, the returned optional will be `.none`.
+  /// Once resolved, the returned optional will be `.some` with the resolved timestamp.
+  public var timestamp: Timestamp? {
+    switch self {
+    case .pending:
+      return .none
+    case let .resolved(timestamp):
+      return .some(timestamp)
+    }
+  }
+
   public init(from decoder: Decoder) throws {
     let container = try decoder.singleValueContainer()
     if container.decodeNil() {