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>

docs/source/providers/inference/remote_nvidia.md

@@ -17,7 +17,7 @@ NVIDIA inference provider for accessing NVIDIA NIM models and AI services.
 
 ```yaml
 url: ${env.NVIDIA_BASE_URL:=https://integrate.api.nvidia.com}
-api_key: ${env.NVIDIA_API_KEY:+}
+api_key: ${env.NVIDIA_API_KEY:=}
 append_api_version: ${env.NVIDIA_APPEND_API_VERSION:=True}
 
 ```

docs/source/providers/inference/remote_runpod.md

@@ -14,8 +14,8 @@ RunPod inference provider for running models on RunPod's cloud GPU platform.
 ## Sample Configuration
 
 ```yaml
-url: ${env.RUNPOD_URL:+}
-api_token: ${env.RUNPOD_API_TOKEN:+}
+url: ${env.RUNPOD_URL:=}
+api_token: ${env.RUNPOD_API_TOKEN:=}
 
 ```
 

docs/source/providers/inference/remote_together.md

@@ -15,7 +15,7 @@ Together AI inference provider for open-source models and collaborative AI devel
 
 ```yaml
 url: https://api.together.xyz/v1
-api_key: ${env.TOGETHER_API_KEY:+}
+api_key: ${env.TOGETHER_API_KEY:=}
 
 ```
 

docs/source/providers/inference/remote_watsonx.md

@@ -17,8 +17,8 @@ IBM WatsonX inference provider for accessing AI models on IBM's WatsonX platform
 
 ```yaml
 url: ${env.WATSONX_BASE_URL:=https://us-south.ml.cloud.ibm.com}
-api_key: ${env.WATSONX_API_KEY:+}
-project_id: ${env.WATSONX_PROJECT_ID:+}
+api_key: ${env.WATSONX_API_KEY:=}
+project_id: ${env.WATSONX_PROJECT_ID:=}
 
 ```