fix empty

Snakefile

@@ -784,21 +784,26 @@ rule count_bases_combined:
         """
         echo {params.header:q} > {output}
         paste_files() {{
+            first_non_empty_file=""
             for file in $@; do
               if [[ -s $file ]]; then
                 first_non_empty_file=$file
                 break
               fi
             done
-            cmd="paste <(cut -f 1-4 $first_non_empty_file)"
-            for file in $@; do
-              if [[ ! -s $file ]]; then
-                cmd+=" <(sed 'c0,0,0,0,0,0,0,0,0,0,,' $first_non_empty_file)"
-              else
-                cmd+=" <(cut -f 5 $file)"
-              fi
-            done
-            eval $cmd
+            if [[ "$first_non_empty_file" != "" ]]; then
+                cmd="paste <(cut -f 1-4 $first_non_empty_file)"
+                for file in $@; do
+                  if [[ ! -s $file ]]; then
+                    cmd+=" <(sed 'c0,0,0,0,0,0,0,0,0,0,,' $first_non_empty_file)"
+                  else
+                    cmd+=" <(cut -f 5 $file)"
+                  fi
+                done
+                eval $cmd
+            else
+                printf ""
+            fi
         }}
         paste_files {input.fwd} >> {output}
         paste_files {input.rev} >> {output}