Allow count_tokens with no contents.

[MarkDaoust]

Fixes: #326

[mayureshagashe2105]

model = genai.GenerativeModel(model_name, system_instruction="Talk in rhymes")
model.count_tokens("")

I think this would still raise the following :

generative-ai-python/google/generativeai/types/content_types.py

Lines 267 to 269 in efead6b

	def to_content(content: ContentType):
	if not content:
	raise ValueError("content must not be empty")

[MarkDaoust]

Hmmm... It goes through to_contentS first, that catches None:

generative-ai-python/google/generativeai/types/content_types.py

Lines 300 to 303 in efead6b

	def to_contents(contents: ContentsType) -> list[glm.Content]:
	if contents is None:
	return []

But to should it let "" through?

Updated title.

[mayureshagashe2105]

It goes through to_contentS first, that catches None

I don't think it will(tested it locally):
"" is not None.
It will go to to_content method and would raise the error. Even if we change to_contents to handle "", something like:

 def to_contents(contents: ContentsType) -> list[glm.Content]: 
     if not contents: 
         return [] 

The request goes through but the API raises error as contents is a required field for the GenerateContentRequest