tab: set homebrew_version in Tab.empty

Library/Homebrew/tab.rb

@@ -168,6 +168,7 @@ class Tab < OpenStruct
 
   def self.empty
     attributes = {
+      "homebrew_version" => HOMEBREW_VERSION,
       "used_options" => [],
       "unused_options" => [],
       "built_as_bottle" => false,

Library/Homebrew/test/tab_test.rb

@@ -33,6 +33,7 @@ class TabTests < Homebrew::TestCase
   def test_defaults
     tab = Tab.empty
 
+    assert_equal HOMEBREW_VERSION, tab.homebrew_version
     assert_empty tab.unused_options
     assert_empty tab.used_options
     assert_nil tab.changed_files