fix 'content' check on messages in chat()

[DSLstandard]

In chat() in Client and AsyncClient, if message['content'] is "",

the following would raise an error since bool("") also returns False like bool(None):

if not message.get('content'):
  raise RequestError('messages must contain content')

Doing the following would fix it:

if 'content' not in message:
  raise RequestError('messages must contain content')

As far as I know ollama does not explicitly prohibit empty content in messages.

[mxyng]

Thanks!