[Auth] Add default values for decoding nil properties in UserInfoImpl (#14185)

Author: ncooke3

FirebaseAuth/CHANGELOG.md

@@ -5,7 +5,13 @@
   `Auth.currentUser` API. This resolves some Firebase 11 issues where the
   current user is unexpectedly `nil` at startup.
 - [fixed] Restore Firebase 10 decoding behavior to prevent user provider data
-  from being decoded as `nil`. (#14011)
+  from always being decoded as `nil` . Note that this fix was only needed for
+  cases where Firebase 11 was reading data written by Firebase 10. (#14011)
+- [fixed] Restore Firebase 10 decoding behavior to prevent user provider data
+  from being decoded as `nil` when a user has multiple linked providers. Note
+  that this fix was only needed for cases where Firebase 11 was reading data
+  written by Firebase 10. Note that this fix will not be in the 11.6.0 zip and
+  Carthage distributions, but will be included from 11.6.0 onwards. (#14011)
 
 # 11.5.0
 - [fixed] Restore pre-Firebase 11 decoding behavior to prevent users getting

FirebaseAuth/Sources/Swift/User/UserInfoImpl.swift

@@ -88,17 +88,16 @@ class UserInfoImpl: NSObject, UserInfo {
   }
 
   required convenience init?(coder: NSCoder) {
-    guard let providerID = coder.decodeObject(
+    let providerID = coder.decodeObject(
       of: [NSString.self],
       forKey: UserInfoImpl.kProviderIDCodingKey
-    ) as? String,
-      let userID = coder.decodeObject(
-        of: [NSString.self],
-        forKey: UserInfoImpl.kUserIDCodingKey
-      ) as? String
-    else {
-      return nil
-    }
+    ) as? String ?? ""
+    // Not all providers have a corresponding user ID (e.g. phone auth), so
+    // fall back to an empty string.
+    let userID = coder.decodeObject(
+      of: [NSString.self],
+      forKey: UserInfoImpl.kUserIDCodingKey
+    ) as? String ?? ""
     let displayName = coder.decodeObject(
       of: [NSString.self],
       forKey: UserInfoImpl.kDisplayNameCodingKey