Ensure box metadata has optional arguments

lib/vagrant/box_metadata.rb

@@ -22,7 +22,7 @@ module Vagrant
     # IO.
     #
     # @param [IO] io An IO object to read the metadata from.
-    def initialize(io)
+    def initialize(io, **_)
       begin
         @raw = JSON.load(io)
       rescue JSON::ParserError => e

plugins/commands/serve/client/box_metadata.rb

@@ -16,7 +16,7 @@ module VagrantPlugins
         # @param [String] version The version to return, this can also
         #   be a constraint.
         # @param [Array<String>] (optional) adds a provider constraint to the version 
-        def version(version, provider)
+        def version(version, provider=[])
           opts = []
           provider.each do |p|
             opts << SDK::BoxMetadata::BoxMetadataOpts.new(name: p)