harmless emails trigger rule EMPTY_MESSAGE, why and how to avoid?

Hello, 

plenty of my emails get tagged by
spamassassin EMPTY_MESSAGE=2.32 although
most of these in turn are very simple
ascii emails without MIME parts or the
like which do have a subject and a
relevant body.

Ironically this also happened to the
welcome email for this very mailinglist:
https://pastebin.com/NegUzr0Y

Any ideas why this happens and how to
correct it?

Further info:

I get email via fetchmail and
filter with procmail which calls spamc.
spamd runs with network rules.

This is on debian trixie and therefore
Spamassassin/spamc/spamd version 4.0.1-5

My modifications to local.cf are:

    allow_user_rules 1
    skip_rbl_checks 0
    dns_server 127.0.0.1 # unbound
    dns_available yes
    
    
    header RCVD_IN_ZEN      eval:check_rbl('zen', 'zen.spamhaus.org.')
    describe RCVD_IN_ZEN    Gelistet in Spamhaus ZEN
    tflags RCVD_IN_ZEN      net
    score RCVD_IN_ZEN       3.5
    

Ciao; Gregor
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