Siri is generally safe. Siri also normally asks for authentication when trying to access contacts etc.
But it has been known for hackers to find ways around this.
Since Siri will ask anyway for authentication to access private data you might aswell stop siri when your noon is locked.
While you on this screen, you could consider the
other options to protect your phone when on locked screen. Do you want notifications, this may show a one time pin (OTP). You also may not want use of the digital wallet.