[25.0] Fix optional unspecified input to conditional step

lib/galaxy/workflow/modules.py

@@ -154,6 +154,8 @@ def to_cwl(
     value, hda_references, step: Optional[WorkflowStep] = None, compute_environment: Optional[ComputeEnvironment] = None
 ):
     element_identifier = None
+    if isinstance(value, NoReplacement):
+        value = None
     if isinstance(value, model.HistoryDatasetCollectionAssociation):
         value = value.collection
     if isinstance(value, model.DatasetCollectionElement):

lib/galaxy_test/workflow/optional_conditional_inputs_to_build_list.gxwf-tests.yml

@@ -0,0 +1,16 @@
+- doc: |
+    Test optional unspecified input into conditional step works
+  job:
+    required_dataset:
+      type: File
+      value: 2.tabular
+      file_type: tabular
+  outputs:
+    out:
+      class: Collection
+      collection_type: list
+      elements:
+        "0":
+          asserts:
+            - has_text:
+                text: "null"

lib/galaxy_test/workflow/optional_conditional_inputs_to_build_list.gxwf.yml

@@ -0,0 +1,25 @@
+class: GalaxyWorkflow
+doc: |
+  This workflow tests the behavior of optional inputs to a conditional database operation step.
+  Regression test for https://github.com/galaxyproject/galaxy/issues/20182
+inputs:
+  required_dataset:
+    type: data
+  optional_dataset:
+    type: data
+    optional: true
+  when:
+    type: boolean
+    default: false
+outputs:
+  out:
+    outputSource: build_list/output
+steps:
+  build_list:
+    tool_id: __BUILD_LIST__
+    in:
+      datasets_0|input: required_dataset
+      datasets_1|input: optional_dataset
+      when:
+        source: when
+    when: $(inputs.when)