Conditionally set SSL certificate request header in Apache
Solution 1:
The example of Header expression shows that the entire parameter has to be quoted ("expr=..." instead of expr="..."). I ended up with
RequestHeader set X-SSL-CLIENT-CERT "%{SSL_CLIENT_CERT}s" "expr=-n %{SSL_CLIENT_CERT}"
and this worked, the "null" string didn't occur anymore (Apache 2.4.6).