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/templates/watsonx/run.yaml

@@ -16,8 +16,8 @@ providers:
     provider_type: remote::watsonx
     config:
       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:=}
   - provider_id: sentence-transformers
     provider_type: inline::sentence-transformers
     config: {}
@@ -85,17 +85,17 @@ providers:
   - provider_id: braintrust
     provider_type: inline::braintrust
     config:
-      openai_api_key: ${env.OPENAI_API_KEY:+}
+      openai_api_key: ${env.OPENAI_API_KEY:=}
   tool_runtime:
   - provider_id: brave-search
     provider_type: remote::brave-search
     config:
-      api_key: ${env.BRAVE_SEARCH_API_KEY:+}
+      api_key: ${env.BRAVE_SEARCH_API_KEY:=}
       max_results: 3
   - provider_id: tavily-search
     provider_type: remote::tavily-search
     config:
-      api_key: ${env.TAVILY_SEARCH_API_KEY:+}
+      api_key: ${env.TAVILY_SEARCH_API_KEY:=}
       max_results: 3
   - provider_id: rag-runtime
     provider_type: inline::rag-runtime