Fix additional_headers fallback in validate_parms_base

[izo0x90]

PR Title

Fix additional_headers fallback in validate_parms_base

PR Description

additional_headers can be passed in explicitly as None in data dict. being processed by validate_parms_base and the .get fallback will not return the intended default value of an empty dict.

A specific usecase where this can be observed is when the VertexAI object is being created by LangChains .configurable_fields/ with_config functionality that allows to specific runtime parameters for steps in the chain.

Example:

    llm = VertexAI(model_name="gemini-pro", project="project-ai").configurable_fields(
        temperature=ConfigurableField(
            id="temperature",
            name="LLM Temperature",
            description="The temperature of the LLM",
        )
    )

    qa_chain = create_stuff_documents_chain(
        llm.with_config(configurable={"temperature": 0.9}),
        TEMPLATE,
        document_prompt=DOC_TEMPLATE,
        output_parser=ANSWER_PARSER,
    )