month value empty?

[reschke]

Describe the issue

Seen in https://www.rfc-editor.org/refs/bibxml2/reference.ANSI.X3-4.1986.xml:

<?xml version="1.0" encoding='UTF-8'?>

<reference anchor="ANSI.X3-4.1986">
<front>
<title>Coded Character Set - 7-bit American Standard Code for Information Interchange</title>
<author>
<organization>American National Standards Institute</organization>
</author>
<date month="" year="1986" />
</front>

<seriesInfo name="ANSI" value="X3.4" />

</reference>

If the month is not known, it should not appear.

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