types/key: add IsZero method to HardwareAttestationKey (#17370)

We will need this for unmarshaling node prefs: use the zero
HardwareAttestationKey implementation when parsing and later check
`IsZero` to see if anything was loaded.

Updates #15830

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

feature/tpm/attestation.go

@@ -262,3 +262,5 @@ func (ak *attestationKey) Clone() key.HardwareAttestationKey {
 		pub:        ak.pub,
 	}
 }
+
+func (ak *attestationKey) IsZero() bool { return !ak.loaded() }