Need validation for missing when value in tool steps

[mvdbeek]

From https://sentry.galaxyproject.org/share/issue/e953d31f25d64b31a8333fc3398a4698/:

reason=<FailureReason.expression_evaluation_failed: 'expression_evaluation_failed'> workflow_step_id=7073867 details=None

I checked, and in this particular case there's a when expression on the step but no input connection. Interestingly, the workflow editor also didn't show the when input as missing (or at all, really). Both backend and frontend should be clear about this being an error condition.