fix bug w/ running input FASTAs w/o any --reference

modules/prokka.nf

@@ -15,7 +15,7 @@ process prokka {
   script:
     """
     # this might be dangerous if the user provides some empty file and THINKS he provided some decent GBK file: this will still work w/o letting the user know
-    if [ -s ${reference} ]; then
+    if [[ -s ${reference} && \$(cat ${reference}) != 'null' ]]; then
       prokka --gcode ${params.gcode} --cpus ${task.cpus} --outdir ${name} --prefix ${name} --proteins ${reference} ${fasta}
     else
       prokka --gcode ${params.gcode} --cpus ${task.cpus} --outdir ${name} --prefix ${name} ${fasta}