fix<ALT>: fixed fault exception when no provider as user/group was defined

! fixed fault exception when no provider as user/group was defined
  closes ALT bug https://bugzilla.altlinux.org/54857

Author: majioa

lib/puppet/type/user.rb

@@ -511,7 +511,7 @@ def insync?(current)
     # @return [Boolean] if the user exists on the system
     # @api private
     def exists?
-      provider.exists?
+      provider&.exists?
     end
 
     newproperty(:roles, :parent => Puppet::Property::List, :required_features => :manages_roles) do