fix: slack empty mention and workflow replies (#300)

*Issue #, if available:*

*Description of changes:*

Fix the below issues:

1. When the slack app is mentioned with an empty message, it throws
Bedrock validation error.
2. When Slack workflow mentiones the app, the reply contains an invalid
mention.


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Author: tmokmss

packages/slack-bolt-app/src/handlers/message.ts

@@ -106,7 +106,7 @@ export async function handleMessage(
               type: 'section',
               text: {
                 type: 'mrkdwn',
-                text: `Hi <@${userId}>, please wait for your agent to launch.\n\n*Useful Tips:*`,
+                text: `${userId ? `Hi <@${userId}>, p` : 'P'}lease wait for your agent to launch.\n\n*Useful Tips:*`,
               },
             },
             {

packages/slack-bolt-app/src/util/history.ts

@@ -11,9 +11,7 @@ export const saveConversationHistory = async (
   imageS3Keys: string[] = []
 ) => {
   const content = [];
-  if (message) {
-    content.push({ text: renderUserMessage({ message }) });
-  }
+  content.push({ text: renderUserMessage({ message }) });
   imageS3Keys.forEach((key) => {
     content.push({
       image: {