Fix additional_headers fallback in validate_parms_base (#700)

Author: izo0x90

libs/vertexai/langchain_google_vertexai/_base.py

@@ -118,7 +118,7 @@ def validate_params_base(cls, values: dict) -> Any:
         if values.get("client_cert_source"):
             client_options.client_cert_source = values["client_cert_source"]
         values["client_options"] = client_options
-        additional_headers = values.get("additional_headers", {})
+        additional_headers = values.get("additional_headers") or {}
         values["default_metadata"] = tuple(additional_headers.items())
         return values