allow references without a date

[dericed]

In our AUTH48 review, we had some references which were undated. mmark adds an empty <date/> element to those references, but the AUTH48 editor removed them rather than adding an undated value.

Currently:

<reference anchor="AVI" target="https://docs.microsoft.com/en-us/windows/win32/directshow/avi-riff-file-reference">
        <front>
          <title>AVI RIFF File Reference</title>
          <author>
            <organization>Microsoft</organization>
          </author>
        </front>
      </reference>

produces

<reference anchor="AVI" target="https://docs.microsoft.com/en-us/windows/win32/directshow/avi-riff-file-reference">
        <front>
          <title>AVI RIFF File Reference</title>
          <author>
            <organization>Microsoft</organization>
          </author>
        </front>
        <date/>
      </reference>

but the empty date element is not needed.