ECMSetupVersion: do not handle SOVERSION value "0" as not set at all

"0" is a fine value, just see ls /usr/lib64/*.so.0.

Ideally would see another unit test to ensure that flaw will not return. No spare time yet, only manually tested.

Guess there might be more of such flawed handling of "0" values in our cmake code, something to check for in some free minutes...