Add test and comment for PATH#existing.

Library/Homebrew/PATH.rb

@@ -59,6 +59,7 @@ class PATH
 
   def existing
     existing_path = select(&File.method(:directory?))
+    # return nil instead of empty PATH, to unset environment variables
     existing_path unless existing_path.empty?
   end
 

Library/Homebrew/test/PATH_spec.rb

@@ -107,5 +107,9 @@ describe PATH do
       expect(path.existing.to_ary).to eq(["/path1"])
       expect(path.to_ary).to eq(["/path1", "/path2"])
     end
+
+    it "returns nil instead of an empty #{described_class}" do
+      expect(described_class.new.existing).to be nil
+    end
   end
 end