ipn/store: make WriteState(id, nil) delete key instead of adding nil entry (#19920)

All StateStore implementations store a nil value in the cache map when WriteState is called with a nil byte slice instead of deleting the key. This causes ReadState to return (nil, nil) instead of (nil, ErrStateNotExist), since the key is still present in the map.

This breaks reset-auth in Windows, Linux, and Android, and the node can't log back in without manually editing the state file. (macOS uses a different state store)
DeleteProfile, DeleteAllProfilesForUser, setUnattendedModeAsConfigured are impacted but don't seem to break because the deleted keys are not reread.

This deletes the key from the cache instead.

Fixes tailscale/corp#42477

Signed-off-by: kari-ts <kari@tailscale.com>

ipn/store.go

@@ -92,7 +92,9 @@ type StateStore interface {
 	// ReadState returns the bytes associated with ID. Returns (nil,
 	// ErrStateNotExist) if the ID doesn't have associated state.
 	ReadState(id StateKey) ([]byte, error)
-	// WriteState saves bs as the state associated with ID.
+	// WriteState saves bs as the state associated with ID. If bs is nil,
+	// the state associated with ID is deleted, and a subsequent ReadState
+	// for ID will return ErrStateNotExist.
 	//
 	// Callers should generally use the ipn.WriteState wrapper func
 	// instead, which only writes if the value is different from what's