Revert "Main executable: only try the CLI version (on Linux and OS X) if there is at least one argument [ci skip]."
Revert "Main executable: only try the CLI version (on Linux and OS X) if there is at least one argument [ci skip]."

This reverts commit 36bfa0be7f92f84015d58f902b4fa616f9a6757f. Indeed,
completely forgot that we can start OpenCOR with some filenames as
arguments!

Fairly/opencor,Fairly/opencor,mirams/opencor,mirams/opencor,Fairly/opencor,mirams/opencor,mirams/opencor,Fairly/opencor,Fairly/opencor,mirams/opencor