chore: delete tests/unit/test_system_prompt_none_bug.py

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"]: