Remove default for optional file

steps/DP3.AOFlaggerStepGenerator.cwl

@@ -28,7 +28,6 @@ inputs:
       path of the input MS is used.
   - id: strategy
     type: File?
-    default: null
     doc: >-
       The name of the strategy file to use. If no name is given, the default
       strategy is used which is fine for HBA. For LBA data the strategy

test_workflow/test.cwl

+class: Workflow
+cwlVersion: v1.0
+id: test
+label: test
+$namespaces:
+  sbg: 'https://www.sevenbridges.com'
+inputs:
+  - id: msin
+    type: Directory
+    'sbg:x': -1111
+    'sbg:y': -277
+outputs:
+  - id: msout
+    outputSource:
+      - dppp/msout
+    type: Directory
+    'sbg:x': -772.234375
+    'sbg:y': -472.50433349609375
+steps:
+  - id: aoflagger_step_generator
+    in: []
+    out:
+      - id: parameters_out
+    run: ../steps/DP3.AOFlaggerStepGenerator.cwl
+    'sbg:x': -1357.640625
+    'sbg:y': -729.5
+  - id: average_step_generator
+    in:
+      - id: input_parset
+        source:
+          - aoflagger_step_generator/parameters_out
+    out:
+      - id: parameters_out
+    run: ../steps/DP3.AveragerStepGenerator.cwl
+    'sbg:x': -1125
+    'sbg:y': -729
+  - id: generic_step
+    in:
+      - id: parameters_input
+        source:
+          - average_step_generator/parameters_out
+    out:
+      - id: output_parset
+      - id: input_files
+      - id: input_directories
+    run: ../steps/DP3.ParsetGenerator.cwl
+    'sbg:x': -1174
+    'sbg:y': -525
+  - id: dppp
+    in:
+      - id: parset
+        source: generic_step/output_parset
+      - id: msin
+        source: msin
+      - id: secondary_files
+        source:
+          - generic_step/input_files
+    out:
+      - id: msout
+    run: ../steps/DPPP.cwl
+    'sbg:x': -957
+    'sbg:y': -530
+requirements:
+  - class: SubworkflowFeatureRequirement