fix(Anthropic): Better handling of whitespace-only content in ChatAnthropic()

[cpsievert]

Closes #10

Also, mirrors the approach from tidyverse/ellmer#393

[Copilot]

Pull Request Overview

This PR improves the handling of whitespace-only responses in ChatAnthropic, closing #10 and mirroring a previous approach.

• Added a test case to verify that whitespace-only responses are handled as expected.
• Updated the ChatAnthropic implementation to replace empty or whitespace-only content with a placeholder.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/test_provider_anthropic.py	Added a new test case (test_anthropic_empty_response) to verify correct handling of whitespace.
chatlas/_anthropic.py	Modified the content block logic to replace empty or whitespace-only responses with a placeholder.

Comments suppressed due to low confidence (2)

chatlas/_anthropic.py:455

• [nitpick] Consider extracting the placeholder value "[empty string]" into a named constant to enhance maintainability and readability.

if text == "" or text.isspace():

tests/test_provider_anthropic.py:94

• [nitpick] Consider adding additional test cases for different whitespace variants (e.g., spaces, tabs) to ensure robust handling of all edge cases.

def test_anthropic_empty_response():