private void validateWorkflow(
            WorkflowDef workflowDef,
            Map<String, Object> workflowInput,
            String externalStoragePath) {
        // Check if the input to the workflow is not null
        if (workflowInput == null && StringUtils.isBlank(externalStoragePath)) {
            LOGGER.error("The input for the workflow '{}' cannot be NULL", workflowDef.getName());
            Monitors.recordWorkflowStartError(
                    workflowDef.getName(), WorkflowContext.get().getClientApp());

            throw new IllegalArgumentException("NULL input passed when starting workflow");
        }
    }
Performs validations for starting a workflow

@throws IllegalArgumentException if the validation fails.
core/src/main/java/com/netflix/conductor/core/operation/StartWorkflowOperation.java