eximconf.erb: standardise on style of condition checks

From the Exim documentation:
"If both strings are omitted, the result is the string true if the
condition is true, and the empty string if the condition is false.
This makes it less cumbersome to write custom ACL and router
conditions."

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>