added optional true to graph related file outputs for spades

Author: tiagofilipe12

flowcraft/generator/templates/spades.nf

@@ -35,8 +35,8 @@ process spades_{{ pid }} {
 
     output:
     set sample_id, file('*_spades*.fasta') into {{ output_channel }}
-    file "*.fastg"
-    file "*.gfa"
+    file "*.fastg" optional true
+    file "*.gfa" optional true
     {% with task_name="spades" %}
     {%- include "compiler_channels.txt" ignore missing -%}
     {% endwith %}