langfuse: value error for usage when streaming is enabled

[Redna]

Describe the bug
When using streaming in the OpenAIGenerator or OpenAIChatgenerator, langfuse will throw an exception that it cannot convert an empty dict into usage input.

site-packages/langfuse/utils/__init__.py", line 101, in _convert_usage_input
    raise ValueError(
ValueError: Usage object must have either {input, output, total, unit} or {promptTokens, completionTokens, totalTokens}
Paris{'tracer': {'name': 'test_langfuse', 'trace_url': 'https://cloud.langfuse.com/trace/4f3dd665-52f1-4938-9882-680dfda94ee8'}, 'llm': {'replies': ['According to the given context, the capital of France is:\n\nParis'], 'meta': [{'model': 'gpt-3.5-turbo', 'index': 0, 'finish_reason': 'stop', 'usage': {}}]}}

To Reproduce

import os

os.environ["TOKENIZERS_PARALLELISM"] = "false"
os.environ["HAYSTACK_CONTENT_TRACING_ENABLED"]="true"
os.environ["LANGFUSE_SECRET_KEY"]="<secret_key>"
os.environ["LANGFUSE_PUBLIC_KEY"]="<public_key>"
os.environ["LANGFUSE_HOST"]="https://cloud.langfuse.com"


from haystack import Pipeline
from haystack.components.retrievers.in_memory import InMemoryBM25Retriever
from haystack.components.builders.prompt_builder import PromptBuilder
from haystack.components.generators import OpenAIGenerator
from haystack.document_stores.in_memory import InMemoryDocumentStore
from haystack import Document

from haystack.utils import Secret

from haystack_integrations.components.connectors.langfuse import LangfuseConnector


def print_callback(data):
    print(data.content, end="")

docstore = InMemoryDocumentStore()
docstore.write_documents([Document(content="Rome is the capital of Italy"), Document(content="Paris is the capital of France")])

query = "What is the capital of France?"

template = """
Given the following information, answer the question.

Context: 
{% for document in documents %}
    {{ document.content }}
{% endfor %}

Question: {{ query }}?
"""
pipe = Pipeline()

pipe.add_component("tracer", LangfuseConnector(name="test_langfuse"))

pipe.add_component("retriever", InMemoryBM25Retriever(document_store=docstore))
pipe.add_component("prompt_builder", PromptBuilder(template=template))
pipe.add_component("llm", OpenAIGenerator(api_key=Secret.from_token("<your-api-key>"),
                                          api_base_url="http://localhost:30091/v1",
                                          streaming_callback=print_callback))

pipe.connect("retriever", "prompt_builder.documents")
pipe.connect("prompt_builder", "llm")

res=pipe.run({
    "prompt_builder": {
        "query": query
    },
    "retriever": {
        "query": query
    }
})

print(res)

Describe your environment (please complete the following information):

• OS: MacOS
• Haystack version: 2.2.1
• Integration version:
  langfuse-haystack 0.0.4