fixed obtaining queue environment variable

conf/vsc_kul_uhasselt.config

@@ -1,9 +1,9 @@
 // Default to /tmp directory if $VSC_SCRATCH scratch env is not available,
 // see: https://github.com/nf-core/configs?tab=readme-ov-file#adding-a-new-config
-scratch_dir   = System.getenv("VSC_SCRATCH") ?: "/tmp"
-tier1_project = System.getenv("SLURM_ACCOUNT") ?: null
-avail_queues  = System.getenv("VSC_DEDICATED_QUEUES") ?: null
-def availQueues = avail_queues?.toString()?.split(',')
+scratch_dir         = System.getenv("VSC_SCRATCH") ?: "/tmp"
+tier1_project       = System.getenv("SLURM_ACCOUNT") ?: null
+def avail_queues    = System.getenv("VSC_DEDICATED_QUEUES") ?: ""
+def availQueues     = avail_queues.toString().split(',')
 
 // Perform work directory cleanup when the run has succesfully completed
 // cleanup = true