Bug Description

When a Feishu user sends a message with empty text content (e.g., {\"text\": \"\"} or a media-only message), the Feishu channel writes a blank user message to the session file:

{ \"role\": \"user\", \"content\": \"\" }

When this session is used in a subsequent LLM request to MiniMax (the default provider), MiniMax rejects the request with:

invalid params, messages must not be empty (2013)

This causes an automatic failover to deepseek-chat, but the MiniMax error still appears in logs for every affected session startup.

Steps to Reproduce

1. Have a Feishu channel configured with MiniMax as the default model
2. Send an empty text message or media-only message through Feishu
3. Observe the error in gateway.err.log

Expected Behavior

Empty-content messages from Feishu should either:

• Be skipped entirely (not written to session)
• Or be replaced with a placeholder like "[Empty message]"

Root Cause

File: dist/extensions/feishu/monitor.account-PVyhNwIK.js, line 92:

function parseMessageContent(content, messageType) {
  if (messageType === \"text\") return parsed.text || \"\";  // ← returns \"\" for empty text
  // ...
}

The empty string flows through handleFeishuMessage → buildFeishuAgentBody → session write → LLM request, with no guard at any layer.

Suggested Fix

In handleFeishuMessage, after parseFeishuMessageEvent() and before writing to session, add a guard:

// Skip empty messages (no text content, no media)
if (!ctx.content.trim() && mediaList.length === 0) {
  log(\`feishu: skipping empty message from \${ctx.senderOpenId}\`);
  return;
}

Alternatively, add a guard in parseMessageContent to return a placeholder for empty content.

Environment

• OpenClaw version: 2026.4.2
• Feishu channel: enabled
• Default model: minimax-portal/MiniMax-M2.7-highspeed
• Frequency: ~40 occurrences over 3 days (4/28–4/30)

Logs

```
2026-04-30T05:27:39.784+08:00 [agent/embedded] embedded run agent end: runId=cc2e2c33-... isError=true model=MiniMax-M2.7-highspeed provider=minimax-portal error=LLM request rejected: invalid params, messages must not be empty (2013)
2026-04-30T02:26:46.318+08:00 [agent/embedded] session file repaired: rewrote 1 assistant message(s), dropped 1 blank user message(s) (819db0af-...)
```