Change way to build the params.outdir_prefix

	Ref: #82

nextflow.config

@@ -108,7 +108,7 @@ params {
 	// Dynamics params, depend on others
 	samplesheet = inputdir.toString() + "/SampleSheet.csv"
 	nf_uniqueness = uniqueness_format.format(new Date())
-	if (outdir_prefix == "") {outdir_prefix = project + "_" + run_name}
+	outdir_prefix = outdir_prefix ?: project + "_" + run_name
 	outdir = inputdir + "/nextflow/" + outdir_prefix + "_" + nf_uniqueness
 
 	subset_seq = miseq_subset_seq