gemini: Fix "invalid argument" error when request contains no tools (#28747)

When we do not have any tools, we want to set the `tools` field to
`None`

Release Notes:

- Fixed an issue where Gemini requests would sometimes return a Bad
Request ("Invalid argument...")

crates/language_models/src/provider/google.rs

@@ -417,7 +417,8 @@ pub fn into_google(
             top_k: None,
         }),
         safety_settings: None,
-        tools: Some(vec![google_ai::Tool {
+        tools: (request.tools.len() > 0).then(|| {
+            vec![google_ai::Tool {
                 function_declarations: request
                     .tools
                     .into_iter()
@@ -427,7 +428,8 @@ pub fn into_google(
                         parameters: tool.input_schema,
                     })
                     .collect(),
-        }]),
+            }]
+        }),
         tool_config: None,
     }
 }