Version.parse: return Version::NULL for unparseable strings

Library/Homebrew/test/testing_env.rb

@@ -36,7 +36,7 @@ module Homebrew
     end
 
     def assert_version_nil(url)
-      assert_nil Version.parse(url)
+      assert Version.parse(url).null?
     end
   end
 

Library/Homebrew/version.rb

@@ -287,7 +287,7 @@ class Version
 
   def self.parse(spec)
     version = _parse(spec)
-    new(version) unless version.nil?
+    version.nil? ? NULL : new(version)
   end
 
   def self._parse(spec)