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/tool_runtime/remote_brave-search.md

@@ -14,7 +14,7 @@ Brave Search tool for web search capabilities with privacy-focused results.
 ## Sample Configuration
 
 ```yaml
-api_key: ${env.BRAVE_SEARCH_API_KEY:+}
+api_key: ${env.BRAVE_SEARCH_API_KEY:=}
 max_results: 3
 
 ```

docs/source/providers/tool_runtime/remote_tavily-search.md

@@ -14,7 +14,7 @@ Tavily Search tool for AI-optimized web search with structured results.
 ## Sample Configuration
 
 ```yaml
-api_key: ${env.TAVILY_SEARCH_API_KEY:+}
+api_key: ${env.TAVILY_SEARCH_API_KEY:=}
 max_results: 3
 
 ```

docs/source/providers/tool_runtime/remote_wolfram-alpha.md

@@ -13,7 +13,7 @@ Wolfram Alpha tool for computational knowledge and mathematical calculations.
 ## Sample Configuration
 
 ```yaml
-api_key: ${env.WOLFRAM_ALPHA_API_KEY:+}
+api_key: ${env.WOLFRAM_ALPHA_API_KEY:=}
 
 ```