Fix crash when a prefix family was empty

hscontrol/db/db.go

@@ -377,16 +377,20 @@ func NewHeadscaleDatabase(
 							}
 						}
 
+						if v4 != nil {
 							err = tx.Model(&types.Node{}).Where("id = ?", node.ID).Update("ipv4", v4.String()).Error
 							if err != nil {
 								return fmt.Errorf("saving ip addresses to new columns: %w", err)
 							}
+						}
 
+						if v6 != nil {
 							err = tx.Model(&types.Node{}).Where("id = ?", node.ID).Update("ipv6", v6.String()).Error
 							if err != nil {
 								return fmt.Errorf("saving ip addresses to new columns: %w", err)
 							}
 						}
+					}
 
 					_ = tx.Migrator().DropColumn(&types.Node{}, "ip_addresses")