SimpleChatTC: Avoid null content, Fix oversight wrt finish_reason

I was wrongly checking for finish_reason to be non null, before
trying to extract the genai content/toolcalls, have fixed this
oversight with the new flow in progress.

I had added few debug logs to identify the above issue, need to
remove them later. Note: given that debug logs are disabled by
replacing the debug function during this program's initialisation,
which I had forgotten about, I didnt get the debug messages and
had to scratch my head a bit, before realising this and the other
issue ;)

Also either when I had originally implemented simplechat 1+ years
back, or later due to changes on the server end, the streaming
flow sends a initial null wrt the content, where it only sets the
role. This was not handled in my flow on the client side, so a
null was getting prepended to the chat messages/responses from the
server. This has been fixed now in the new generic flow.

tools/server/public_simplechat/simplechat.js

@@ -155,6 +155,10 @@ class SimpleChat {
      * @param {{key: string, value: string}} resp
      */
     append_response(resp) {
+        if (resp.value == null) {
+            return
+        }
+        console.debug(resp.key, resp.value)
         this.latestResponse[resp.key] += resp.value;
     }
 
@@ -311,10 +315,11 @@ class SimpleChat {
      * @param {string} apiEP
      */
     response_extract_stream(respBody, apiEP) {
+        console.debug(respBody, apiEP)
         let key = "content"
         let assistant = "";
         if (apiEP == ApiEP.Type.Chat) {
-            if (respBody["choices"][0]["finish_reason"] !== null) {
+            if (respBody["choices"][0]["finish_reason"] === null) {
                 if (respBody["choices"][0]["delta"]["content"] !== undefined) {
                     assistant = respBody["choices"][0]["delta"]["content"];
                 } else {