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/postgres-demo/postgres_demo.py

@@ -52,7 +52,7 @@ def get_distribution_template() -> DistributionTemplate:
         Provider(
             provider_id="${env.ENABLE_CHROMADB:+chromadb}",
             provider_type="remote::chromadb",
-            config=ChromaVectorIOConfig.sample_run_config(url="${env.CHROMADB_URL:+}"),
+            config=ChromaVectorIOConfig.sample_run_config(url="${env.CHROMADB_URL:=}"),
         ),
     ]
     default_tool_groups = [
@@ -114,7 +114,7 @@ def get_distribution_template() -> DistributionTemplate:
                             provider_id="meta-reference",
                             provider_type="inline::meta-reference",
                             config=dict(
-                                service_name="${env.OTEL_SERVICE_NAME:+}",
+                                service_name="${env.OTEL_SERVICE_NAME:=}",
                                 sinks="${env.TELEMETRY_SINKS:=console,otel_trace}",
                                 otel_trace_endpoint="${env.OTEL_TRACE_ENDPOINT:=http://localhost:4318/v1/traces}",
                             ),

llama_stack/templates/postgres-demo/run.yaml

@@ -23,7 +23,7 @@ providers:
   - provider_id: ${env.ENABLE_CHROMADB:+chromadb}
     provider_type: remote::chromadb
     config:
-      url: ${env.CHROMADB_URL:+}
+      url: ${env.CHROMADB_URL:=}
   safety:
   - provider_id: llama-guard
     provider_type: inline::llama-guard
@@ -51,19 +51,19 @@ providers:
   - provider_id: meta-reference
     provider_type: inline::meta-reference
     config:
-      service_name: ${env.OTEL_SERVICE_NAME:+}
+      service_name: ${env.OTEL_SERVICE_NAME:=}
       sinks: ${env.TELEMETRY_SINKS:=console,otel_trace}
       otel_trace_endpoint: ${env.OTEL_TRACE_ENDPOINT:=http://localhost:4318/v1/traces}
   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