Bug: DocIndex search fails when it's empty

[jupyterjazz]

import numpy as np
from docarray import BaseDoc
from docarray.index import InMemoryExactNNIndex
from docarray.typing import NdArray
from pydantic import parse_obj_as


class MyDoc(BaseDoc):
    emb: NdArray


index = InMemoryExactNNIndex[MyDoc]()
query = parse_obj_as(NdArray, np.random.rand(5))
docs, _ = index.find(query, search_field="emb", limit=10)

ValueError: need at least one array to stack

[AnneYang720]

I think for this bug, we should just add judgment in find() (and find_batched()) of InMemoryExactNNIndex.
I prefer not to change any methods (probably _extract_embeddings() and more) in utils, because they may be used elsewhere and their type hints already say the index field is not optional.
what do u think @JohannesMessner @samsja