fix: allow default empty vars for conditionals (#2570)

# What does this PR do?

We were not using conditionals correctly, conditionals can only be used
when the env variable is set, so `${env.ENVIRONMENT:+}` would return
None is ENVIRONMENT is not set.

If you want to create a conditional value, you need to do
`${env.ENVIRONMENT:=}`, this will pick the value of ENVIRONMENT if set,
otherwise will return None.

Closes: https://github.com/meta-llama/llama-stack/issues/2564

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/providers/remote/inference/together/config.py

@@ -26,5 +26,5 @@ class TogetherImplConfig(BaseModel):
     def sample_run_config(cls, **kwargs) -> dict[str, Any]:
         return {
             "url": "https://api.together.xyz/v1",
-            "api_key": "${env.TOGETHER_API_KEY:+}",
+            "api_key": "${env.TOGETHER_API_KEY:=}",
         }