Google workflow does not directly accept ternary operators. To implement this you can add a step that uses a switch block.
see more here:
https://cloud.google.com/workflows/docs/reference/syntax/conditions#yaml

- check_some_variable_exists:
        switch:
          - condition: ${"some_variable" in input}
            assign:
              - some_variable: ${input.some_variable}
          - condition: ${true}
            assign:
              - some_variable: "default_value"