Add Registry#empty? to check if a registry has any items

lib/vagrant/registry.rb

@@ -59,6 +59,13 @@ module Vagrant
     end
     alias_method :size, :length
 
+    # Checks if this registry has any items.
+    #
+    # @return [Boolean]
+    def empty?
+      @items.keys.empty?
+    end
+
     # Merge one registry with another and return a completely new
     # registry. Note that the result cache is completely busted, so
     # any gets on the new registry will result in a cache miss.

test/unit/vagrant/registry_test.rb

@@ -116,6 +116,17 @@ describe Vagrant::Registry do
     end
   end
 
+  describe "#empty" do
+    it "should return true when the registry is empty" do
+      expect(instance.empty?).to be(true)
+    end
+
+    it "should return false when there is at least one element" do
+      instance.register("foo") { }
+      expect(instance.empty?).to be(false)
+    end
+  end
+
   describe "merging" do
     it "should merge in another registry" do
       one = described_class.new