Fix last_process related bug in RPC.health

freqtrade/rpc/api_server/api_schemas.py

@@ -456,5 +456,5 @@ class SysInfo(BaseModel):
 
 
 class Health(BaseModel):
-    last_process: datetime
-    last_process_ts: int
+    last_process: Optional[datetime]
+    last_process_ts: Optional[int]

freqtrade/rpc/api_server/api_v1.py

@@ -346,4 +346,4 @@ def sysinfo():
 
 @router.get('/health', response_model=Health, tags=['info'])
 def health(rpc: RPC = Depends(get_rpc)):
-    return rpc._health()
+    return rpc.health()