fix(firebase_auth): Check if UserMetadata properties are null before parsing. (#8313)

Author: Ehesp

packages/firebase_auth/firebase_auth_web/lib/src/firebase_auth_web_user.dart

@@ -25,13 +25,17 @@ class UserWeb extends UserPlatform {
           'email': _webUser.email,
           'emailVerified': _webUser.emailVerified,
           'isAnonymous': _webUser.isAnonymous,
-          'metadata': <String, int>{
-            'creationTime': _dateFormat
-                .parse(_webUser.metadata.creationTime)
-                .millisecondsSinceEpoch,
-            'lastSignInTime': _dateFormat
-                .parse(_webUser.metadata.lastSignInTime)
-                .millisecondsSinceEpoch,
+          'metadata': <String, int?>{
+            'creationTime': _webUser.metadata.creationTime != null
+                ? _dateFormat
+                    .parse(_webUser.metadata.creationTime!)
+                    .millisecondsSinceEpoch
+                : null,
+            'lastSignInTime': _webUser.metadata.lastSignInTime != null
+                ? _dateFormat
+                    .parse(_webUser.metadata.lastSignInTime!)
+                    .millisecondsSinceEpoch
+                : null,
           },
           'phoneNumber': _webUser.phoneNumber,
           'photoURL': _webUser.photoURL,

packages/firebase_auth/firebase_auth_web/lib/src/interop/auth_interop.dart

@@ -344,11 +344,11 @@ abstract class ActionCodeInfo {
 abstract class UserMetadata {
   /// The date the user was created, formatted as a UTC string.
   /// For example, 'Fri, 22 Sep 2017 01:49:58 GMT'.
-  external String get creationTime;
+  external String? get creationTime;
 
   /// The date the user last signed in, formatted as a UTC string.
   /// For example, 'Fri, 22 Sep 2017 01:49:58 GMT'.
-  external String get lastSignInTime;
+  external String? get lastSignInTime;
 }
 
 /// A structure for [User]'s user profile.