Reject null input in agent tool parameter validation

JSON null unmarshals into an empty string, so the presence-only
key check let {"input":null} through, running the nested agent
with a blank user message.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/agent/agent_tool.go

@@ -83,7 +83,8 @@ func (t *agentTool) Execute(ctx context.Context, arguments string) (ToolResult,
 		}, nil
 	}
 
-	if _, ok := fields["input"]; !ok {
+	raw, ok := fields["input"]
+	if !ok || string(raw) == "null" {
 		return ToolResult{
 			Content: "Missing required parameters: input",
 			IsError: true,