kernel/v2: not an error if synced folder guest path is empty [GH-4833]

plugins/kernel_v2/config/vm.rb

@@ -603,6 +603,7 @@ module VagrantPlugins
           guestpath = Pathname.new(options[:guestpath])
           hostpath  = Pathname.new(options[:hostpath]).expand_path(machine.env.root_path)
 
+          if guestpath.to_s != ""
             if guestpath.relative? && guestpath.to_s !~ /^\w+:/
               errors << I18n.t("vagrant.config.vm.shared_folder_guestpath_relative",
                                path: options[:guestpath])
@@ -614,6 +615,7 @@ module VagrantPlugins
 
               used_guest_paths.add(options[:guestpath])
             end
+          end
 
           if !hostpath.directory? && !options[:create]
             errors << I18n.t("vagrant.config.vm.shared_folder_hostpath_missing",

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

@@ -480,6 +480,12 @@ describe VagrantPlugins::Kernel_V2::VMConfig do
       expect(sf["/vagrant"][:disabled]).to be_false
       expect(sf["/vagrant"][:foo]).to eq(:bar)
     end
+
+    it "is not an error if guest path is empty" do
+      subject.synced_folder(".", "")
+      subject.finalize!
+      assert_valid
+    end
   end
 
   describe "#usable_port_range" do