Do not add system prompt part when dynamic system prompt function returns empty value

[giacbrd]

The system_prompt decorator allows to dynamically add parts to the messages. However, a part generated by a decorated function is always added, even if the function output is useless, e.g. an empty string. Sometime the dynamic behaviour we need is to just increment the system prompt, according to the context.

This PR add this behaviour by:

• avoiding adding a system prompt part in the messages if a system prompt function returns a value that evaluates to False
• defining the return type of these functions as str | None, so that it is explicit that these function can optionally return a value

Example of a dynamic system prompt where we add some information only if the retrieve call is successful

@agent.system_prompt
async def get_more_information(ctx: RunContext[MyDeps]) -> str | None:
    response = await ctx.deps.http_client.get(
        'https://example.com',
        headers={'Authorization': f'Bearer {ctx.deps.api_key}'},
    )
    if response.status_code == 200 and response.text.strip():
        return f'You also have this information: {response.text}'
    return None

[giacbrd]

The use of Union[str, None] instead of str | None in most of the type hints is due to compatibility with python 3.9, which fails to run the new code otherwise