ChatAnthropic fails to handle empty HumanMessage content, leading to BadRequestError (400) from Anthropic API

### Checked other resources - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [x] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs _No response_ ### Reproduction Steps / Example Code (Python) ```python from langchain.chat_models import init_chat_model from langchain_core.messages import HumanMessage, AIMessage, SystemMessage import os llm = init_chat_model( model="claude-haiku-3", api_key=os.getenv('ANTHROPIC_API_KEY'), max_retries=0, max_tokens=100 ) print(type(llm)) messages = [ HumanMessage(content="", source="Alice"), ] response = llm.invoke(messages) print(response) ``` ### Error Message and Stack Trace (if applicable) ```shell --------------------------------------------------------------------------- BadRequestError Traceback (most recent call last) Cell In[14], line 18 12 print(type(llm)) 14 messages = [ 15 HumanMessage(content="", source="Alice"), 16 ] ---> 18 response = llm.invoke(messages) 1…