fix: guard against None system_prompt in _ensure_persona_and_skills

ProviderRequest.system_prompt defaults to None. When a persona with a
prompt is configured, _ensure_persona_and_skills calls
``req.system_prompt += ...`` which crashes with ``TypeError`` when
system_prompt is None.

Added a None guard before the persona prompt injection and skills prompt
appending sections so they always operate on a string.

astrbot/core/astr_main_agent.py

@@ -399,6 +399,9 @@ async def _ensure_persona_and_skills(
         event, extract_persona_custom_error_message_from_persona(persona)
     )
 
+    if req.system_prompt is None:
+        req.system_prompt = ""
+
     if persona:
         # Inject persona system prompt
         if prompt := persona["prompt"]:

tests/unit/test_system_prompt_none_bug.py

@@ -0,0 +1,51 @@
+"""Test that _ensure_persona_and_skills handles None system_prompt.
+
+When ProviderRequest.system_prompt is None (the default), calling
+_ensure_persona_and_skills with a persona that has a prompt should
+not crash with ``TypeError: unsupported operand type(s) for +=``.
+
+The bug was that ``req.system_prompt += ...`` was called when
+system_prompt was None instead of initializing it to "" first.
+"""
+
+import pytest
+from unittest.mock import AsyncMock, MagicMock
+
+
+@pytest.mark.asyncio
+async def test_ensure_persona_and_skills_handles_none_system_prompt():
+    from astrbot.core.astr_main_agent import _ensure_persona_and_skills
+    from astrbot.core.provider.entities import ProviderRequest
+    from astrbot.core.db.po import ConversationV2
+
+    # ProviderRequest.system_prompt defaults to None
+    req = ProviderRequest()
+    req.conversation = ConversationV2(
+        conversation_id="test",
+        platform_id="test",
+        user_id="test",
+    )
+
+    cfg = {"computer_use_runtime": "local"}
+
+    plugin_context = MagicMock()
+    plugin_context.persona_manager.resolve_selected_persona = AsyncMock(
+        return_value=(
+            "persona-1",
+            {"prompt": "You are a helpful assistant.", "name": "test"},
+            None,
+            False,
+        )
+    )
+
+    event = MagicMock()
+    event.unified_msg_origin = "test:test_user"
+
+    # This should NOT raise TypeError even though req.system_prompt is None
+    try:
+        await _ensure_persona_and_skills(req, cfg, plugin_context, event)
+    except TypeError as e:
+        pytest.fail(f"_ensure_persona_and_skills crashed with TypeError: {e}")
+
+    assert req.system_prompt is not None
+    assert "# Persona Instructions" in req.system_prompt