core: Vagrant.server_url should default if empty

lib/vagrant/shared_helpers.rb

@@ -19,7 +19,9 @@ module Vagrant
   #
   # @return [String]
   def self.server_url
-    ENV["VAGRANT_SERVER_URL"] || DEFAULT_SERVER_URL
+    result = ENV["VAGRANT_SERVER_URL"]
+    result = nil if result == ""
+    result || DEFAULT_SERVER_URL
   end
 
   # The source root is the path to the root directory of the Vagrant source.

test/unit/vagrant/shared_helpers_test.rb

@@ -30,6 +30,13 @@ describe Vagrant do
       end
     end
 
+    it "defaults if the string is empty" do
+      with_temp_env("VAGRANT_SERVER_URL" => "") do
+        expect(subject.server_url).to eq(
+          Vagrant::DEFAULT_SERVER_URL)
+      end
+    end
+
     it "is the VAGRANT_SERVER_URL value" do
       with_temp_env("VAGRANT_SERVER_URL" => "foo") do
         expect(subject.server_url).to eq("foo")