BUG/MINOR: tools: always terminate empty lines

BUG/MINOR: tools: always terminate empty lines

Since latest commit 7e4a2f39ef ("BUG/MINOR: tools: do not create an empty
arg from trailing spaces"), an empty line will no longer produce an arg
and no longer append a trailing zero to them. This was not visible because
one is already present in the input string, however all the trailing args
are set to out+outpos-1, which now points one char before the buffer since
nothing was emitted, and was noticed by ASAN, and/or when parsing garbage.
Let's make sure to always emit the zero for empty lines as well to address
this issue. No backport is needed unless the patch above gets backported.