Treat an empty box value as invalid

This fixes an issue where having a box name set to an empty string will
cause all Vagrant commands to fail with an error like:

ArgumentError: Malformed version number string (random box name)

This may be related to #10663.

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 = nil if @box == UNSET_VALUE || @box.to_s.empty?
         @ignore_box_vagrantfile = false if @ignore_box_vagrantfile == UNSET_VALUE
 
         if @box_check_update == UNSET_VALUE

test/unit/plugins/kernel_v2/config/vm_test.rb

@@ -86,6 +86,12 @@ describe VagrantPlugins::Kernel_V2::VMConfig do
       assert_invalid
     end
 
+    it "cannot be an empty string" do
+      subject.box = ""
+      subject.finalize!
+      assert_invalid
+    end
+
     it "is not required if the provider says so" do
       machine.provider_options[:box_optional] = true
       subject.box = nil