Allow empty strings in workflow_dispatch choice options (#245)

Fixes vscode#395 - Empty value for choice option shows 'Unexpected value' error

Empty strings are valid options for workflow_dispatch inputs with type: choice.
They allow users to make a choice 'optional' or force explicit selection.

Changes:
- Add sequence-of-string type that allows empty strings (unlike sequence-of-non-empty-string)
- Use sequence-of-string for workflow_dispatch options field
- Add test to verify empty string in choice options doesn't produce validation errors

workflow-parser/src/workflow-v1.0.json

@@ -1539,7 +1539,7 @@
           },
           "default": "workflow-dispatch-input-default",
           "options": {
-            "type": "sequence-of-non-empty-string",
+            "type": "sequence-of-string",
             "description": "The options of the dropdown list, if the type is a choice."
           }
         }
@@ -2419,6 +2419,11 @@
         "item-type": "non-empty-string"
       }
     },
+    "sequence-of-string": {
+      "sequence": {
+        "item-type": "string"
+      }
+    },
     "boolean-needs-context": {
       "context": [
         "github",