* fix(proxy_server.py): only update k,v pair if v is not empty/null
Fixes https://github.com/BerriAI/litellm/issues/6787
* test(test_router.py): cleanup duplicate calls
* test: add new test stream options drop params test
* test: update optional params / stream options test to test for vertex ai mistral route specifically
Addresses https://github.com/BerriAI/litellm/issues/7309
* fix(proxy_server.py): fix linting errors
* fix: fix linting errors