flutter check empty object

Summary
This answer holds true, except for isNull and isNotNull. They no longer provide type promotion when Null Safety is introduced in dart/flutter in the future.
Other helpers like isNullOrEmpty do not provide type promotion, as they are in a different (sub-)scope compared to callsite.
My personal opinion, is that you can drop isNull and isNotNull but keep other helpers as you shouldn't expect them to do type promotion for you.
Explanation
This is because Null Safety was finally introduced in Dart/Flutter. But as of October 2020, this feature is still not available for stable releases on dart/flutter. Check out the quick guide Null Safety or the thorough Understanding Null Safety.
Null Safety in Dart/Flutter should be similar to Swift (Optional) and Kotlin (Nullable Types, Non-Nullable Types).
Demonstration
Here's a demonstration why encapsulation/helper-getter of isNull (== null) and isNotNull (!= null) is a very big problem:

// Promotion works

int definitelyInt(int? aNullableInt) {
  if (aNullableInt == null) { // Promote variable `aNullableInt` of Nullable type `int?` to Non-Nullable type `int`
    return 0;
  }
  return aNullableInt; // Can't be null! This variable is promoted to non-nullable type `int`
}
When "Null Safety" is shipped in the dart release you are using, the type promotion in above code works! HOWEVER:

// Promotion does NOT work!!!

int definitelyInt(int? aNullableInt) {
  if (aNullableInt.isNull) { // does NOT promote variable `aNullableInt` of Nullable type `int?`
    return 0;
  }
  return aNullableInt; // This variable is still of type `int?`!!!
}

The above doesn't work, because the null check == null and != null are encapsulated in a sub-scope (different stack frame) and not inferred to have this "promotion" effect within definitelyInt scope.