Fix plugin discovery loading when no data directory is available

If the local data directory is unavailable, there will be no local
file to use within the plugin manager. Check for local file before
returning result, otherwise just return an empty hash.

lib/vagrant/environment.rb

@@ -1021,7 +1021,11 @@ module Vagrant
         ui.warn(I18n.t("vagrant.plugins.local.install_rerun_command"))
         exit(-1)
       end
-      Vagrant::Plugin::Manager.instance.local_file.installed_plugins
+      if Vagrant::Plugin::Manager.instance.local_file
+        Vagrant::Plugin::Manager.instance.local_file.installed_plugins
+      else
+        {}
+      end
     end
 
     # This method copies the private key into the home directory if it

test/unit/vagrant/environment_test.rb

@@ -1561,10 +1561,18 @@ VF
       allow(plugin_manager).to receive(:load_plugins)
     end
 
+    context "when local data directory does not exist" do
+      let(:local_file) { nil }
+
+      it "should properly return empty result" do
+        expect(instance.send(:process_configured_plugins)).to be_empty
+      end
+    end
+
     context "plugins are disabled" do
       before{ allow(Vagrant).to receive(:plugins_enabled?).and_return(false) }
 
-      it "should return nil" do
+      it "should return empty result" do
         expect(instance.send(:process_configured_plugins)).to be_nil
       end
     end