🐛 [meidiapipe_text] Null check failed by cosineSimilarity

OS: Apple Silicon(ARM) Mac OS
Test Device: Android Emulator

It seems like the embedding pointer becomes null If It is not created from the embedder.

```
final textEumbedResult = await embedder.embed(text);

final embedding = Embedding.float(
Float32List.fromList(te),
headIndex: 1,
);

final similarity = await embedder.cosineSimilarity(
textEumbedResult.embeddings.first,
embedding,
);

print("similarity: $similarity");
```

E/flutter ( 6844): Null check operator used on a null value
E/flutter ( 6844): #0 _embedderService (package:mediapipe_text/src/io/tasks/text_embedding/text_embedder.dart:124:55)

```
case _EmbedderTaskType._cosineSimilarity:
p.send(
executor.cosineSimilarity(
(message.a as core_io.Embedding).pointer!,
(message.b as core_io.Embedding).pointer!,
),
);
```