fix: type 'Null' is not a subtype of type 'String' in getCurrentUser (#…

…46)

It was reported that the mParticle SDK has a null pointer bug when trying to retrieve the mpid on some Android devices.  This commit adds a null check for the mpid before attempting to return a User.

lib/mparticle_flutter_sdk.dart

@@ -182,11 +182,11 @@ class MparticleFlutterSdk {
   }
 
   Future<User?> getCurrentUser() async {
-    String mpid = await _channel.invokeMethod('getMPID');
-    if (mpid == "") {
-      return null;
+    String? mpid = await _channel.invokeMethod('getMPID');
+    if (mpid != null && mpid.isNotEmpty) {
+      return new User(mpid);
     }
-    return new User(mpid);
+    return null;
   }
 
   Future<Map> getAttributions() async {