Requesting qcstats when none exist fails

[unode]

Given:

ngless "1.0"
import "parallel" version "0.6"

samples = readlines("samplefile")
sample = lock1(samples)
input = samfile(sample + ".sam.gz")

collect(qcstats({fastq}), ofile="fqstats.txt", current=sample, allneeded=samples)
collect(qcstats({mapping}), ofile="mapstats.txt", current=sample, allneeded=samples)

This fails with:

[Thu 04-07-2019 13:47]: Script OK. Starting interpretation...
[Thu 04-07-2019 13:47] Line 5: lock1: Obtained lock file: 'ngless-locks/35ca1240/sample1.lock'
[Thu 04-07-2019 13:47] Line 5: Writing stats to 'ngless-stats/35ca1240/sample1'
Exiting after fatal error while loading and running script
Should Not Occur Error! This probably indicates a bug in ngless.
	Please get in touch with the authors with a description of how this happened.
	If possible run your script with the --trace flag and post the script and the resulting trace at 
		https://github.com/ngless-toolkit/ngless/issues
	or email us at coelho@embl.de.
Trying to merge a headerless file

It should just skip, create an empty file or fail with a better error message.