SimpleChatTC: Saner/Robust AssistantResponse content_equiv

Previously if content was empty, it would have always sent the
toolcall info related version even if there was no toolcall info
in it. Fixed now to return empty string, if both content and
toolname are empty.

tools/server/public_simplechat/simplechat.js

@@ -71,8 +71,10 @@ class AssistantResponse {
     content_equiv() {
         if (this.response.content !== "") {
             return this.response.content;
-        } else {
+        } else if (this.response.toolname !== "") {
             return `ToolCall:${this.response.toolname}:${this.response.toolargs}`;
+        } else {
+            return ""
         }
     }