fix: allow local embeddings in EmbeddingStrategy

[majiayu000]

Summary

Fixes #1658

The _get_embedding_llm_config_dict method was always returning a fallback OpenAI configuration when no embedding_llm_config was provided. This made local sentence-transformers embeddings unreachable when using AdaptiveCrawler.

List of files changed and why

• crawl4ai/adaptive_crawler.py - Changed _get_embedding_llm_config_dict to return None instead of a fallback dict, allowing get_text_embeddings() to use the local sentence-transformers path when the user intends to use local embeddings
• tests/unit/test_embedding_strategy_local.py - Added unit test to verify the fix

How Has This Been Tested?

• Unit test verifies _get_embedding_llm_config_dict() returns None when no config provided
• Test uses AST parsing to avoid import dependency issues

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes