Add validation error for empty box

This commit adds a unique error message for an empty box value. It
requires modifications to vagrantfile.rb because some Vagrantfile config
is used before validation occurs.

lib/vagrant/vagrantfile.rb

@@ -197,7 +197,7 @@ module Vagrant
         local_keys = keys.dup
 
         # Load the box Vagrantfile, if there is one
-        if config.vm.box && boxes
+        if !config.vm.box.to_s.empty? && boxes
           box = boxes.find(config.vm.box, box_formats, config.vm.box_version)
           if box
             box_vagrantfile = find_vagrantfile(box.directory)

plugins/kernel_v2/config/vm.rb

@@ -511,7 +511,7 @@ module VagrantPlugins
         @base_mac = nil if @base_mac == UNSET_VALUE
         @base_address = nil if @base_address == UNSET_VALUE
         @boot_timeout = 300 if @boot_timeout == UNSET_VALUE
-        @box = nil if @box == UNSET_VALUE || @box.to_s.empty?
+        @box = nil if @box == UNSET_VALUE
         @ignore_box_vagrantfile = false if @ignore_box_vagrantfile == UNSET_VALUE
 
         if @box_check_update == UNSET_VALUE
@@ -732,6 +732,10 @@ module VagrantPlugins
           errors << I18n.t("vagrant.config.vm.clone_and_box")
         end
 
+        if box && box.empty?
+          errors << I18n.t("vagrant.config.vm.box_empty")
+        end
+
         errors << I18n.t("vagrant.config.vm.hostname_invalid_characters", name: machine.name) if \
           @hostname && @hostname !~ /^[a-z0-9][-.a-z0-9]*$/i
 

templates/locales/en.yml

@@ -1980,6 +1980,7 @@ en:
           Checksum type specified but "box_download_checksum" is blank
         box_download_checksum_notblank: |-
           Checksum specified but must also specify "box_download_checksum_type"
+        box_empty: "Box value is an empty string."
         box_missing: "A box must be specified."
         box_download_options_type: |-
           Found "box_download_options" specified as type '%{type}', should be a Hash

test/unit/vagrant/vagrantfile_test.rb

@@ -368,6 +368,17 @@ describe Vagrant::Vagrantfile do
         to raise_error(Vagrant::Errors::ProviderNotUsable)
     end
 
+    it "does not try to load the box if the box is empty" do
+      provider_cls = register_provider("foo")
+
+      configure do |config|
+        config.vm.box = ""
+      end
+
+      expect(boxes).not_to receive(:find)
+      results = subject.machine_config(:default, :foo, boxes)
+    end
+
     context "when provider validation is ignored" do
       before do
         configure do |config|