BUG/MINOR: config: fix arg number reported on empty arg warning

BUG/MINOR: config: fix arg number reported on empty arg warning

If an empty argument is used in configuration, for example due to an
undefined environment variable, the rest of the line is not parsed. As
such, a warning is emitted to report this.

The warning was not totally correct as it reported the wrong argument
index. Fix this by this patch. Note that there is still an issue with
the "^" indicator, but this is not as easy to fix yet.

This is related to github issue #2995.

This should be backported up to 3.2.