Content fields break the alternating table row color for {allelements}

[kraftner]

Issue Overview

The alternating grey/white color for the table rows the {allelements} tag creates breaks if "Content" fields (or any other fields that don't have data) are used.

Steps to Reproduce (for bugs)

1. Create a form with multiple fields, one of them (not the last) being of type Content.
2. Create a mail notification that uses the {allelements} tag.
3. Notice that the field before and after the Content field have the some background color in the generated HTML table in the mail.

Expected Behavior

The row colors always alternate, no matter what fields are used.

Current Behavior

They don't.

Possible Solution

Fields with no output should not increase the counter.