Ensure empty message values are properly formatted

lib/vagrant/ui.rb

@@ -329,12 +329,15 @@ module Vagrant
         target = opts[:target] if opts.key?(:target)
         target = "#{target}:" if target != ""
 
-        lines = [].tap do |l|
-          message.scan(/(.*?)(\n|$)/).each do |m|
-            l << m.first if m.first != "" || (m.first == "" && m.last == "\n")
+        lines = [message]
+        if message != ""
+          lines = [].tap do |l|
+            message.scan(/(.*?)(\n|$)/).each do |m|
+              l << m.first if m.first != "" || (m.first == "" && m.last == "\n")
+            end
           end
+          lines << "" if message.end_with?("\n")
         end
-        lines << "" if message.end_with?("\n")
 
         # Otherwise, make sure to prefix every line properly
         lines.map do |line|

test/unit/vagrant/ui_test.rb

@@ -413,5 +413,10 @@ describe Vagrant::UI::Prefixed do
         expect(subject.format_message(:detail, msg).count("\n")).to eq(msg.count("\n"))
       end
     end
+
+    it "should properly format a blank message" do
+      expect(subject.format_message(:detail, "", target: "default", prefix: true)).
+        to match(/\s+default:\s+/)
+    end
   end
 end