reftest engine: empty arguments are not taken into account on Windows

[kit-ty-kate]

### <test.sh>
x=$1
echo "$1"
### sh test.sh ""

Gives different results in Unix vs. Windows and should probably be investigated at some point.
The former gives out the expected empty string and the latter is non-empty (i don't have time to test further but this is reproducible with update.test in a1d9f53)