fix(cartesia-sdk): treat empty profile as no memories, not a retrieval error

[Cintu07]

A user with no stored memories gets profile=None from the API. _retrieve_memories
reads response.profile.static/.dynamic without a None check, so it raises
AttributeError for those users. The surrounding try/except turns that into
MemoryRetrievalError, which the caller logs as "Memory retrieval failed" and then
skips memory injection.

So every new or empty-profile user triggers a false error log each turn, and the
empty case is treated as a failure instead of "no memories yet". It also makes
real retrieval failures (timeouts, API errors) indistinguishable from the normal
empty case in logs.

The pipecat SDK already handles this after #1027/#1033 by treating a None profile
as empty memories. The cartesia SDK has the same unguarded access and was missed.

Changes:

• return empty static/dynamic when profile is None instead of dereferencing it
• search results handling and logging unchanged
• add a regression test for the None-profile response, modeled on pipecat's
  test_empty_profile.py

Tested:
PYTHONPATH=src python -m unittest tests.test_empty_profile
(passes; fails on the original code with 'NoneType' object has no attribute 'static')

[vorflux]

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