fix(utils): embed(texts=[]) crashes with IndexError on empty input

[devteamaegis]

What's broken

Client.embed(texts=[]) (default batching, no images) produces an empty responses list, which merge_embed_responses indexes unconditionally:

>>> import cohere
>>> co = cohere.Client(api_key="fake-no-network")
>>> co.embed(texts=[])
IndexError: list index out of range   # src/cohere/utils.py, responses[0].response_type

The crash happens before any network I/O, so an empty input list never gets a meaningful error.

Why it happens

merge_embed_responses reads responses[0] without checking for an empty list.

Fix

Guard the empty case and raise a clear ValueError. The SDK can't synthesise a typed empty EmbedResponse (it wouldn't know response_type), so an explicit, actionable error is the smallest correct behavior and matches the library's existing input-validation pattern.

Test

Added a case asserting merge_embed_responses([]) raises ValueError instead of IndexError.

Fixes #777

---

Note

Low Risk
Small input-validation guard and test only; no API or embedding merge logic changes for non-empty input.

Overview
Fixes Client.embed(texts=[]) (and any path that merges zero batched responses) so it no longer crashes with IndexError when merge_embed_responses accessed responses[0].

merge_embed_responses now rejects an empty responses list up front with a ValueError and an explicit message that no texts were provided. A unit test locks in that behavior instead of an IndexError.

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