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
eric sciple