Add @empty modifier to workflow steps

[fractlrao]

A common pattern is applications is below:

    {ScalarSetting {SettingKey? "planning_week_weights"}} @as [PlanningWeightsSetting]
    if (PlanningWeightsSetting.length == 0) {
        helper.getDefaultPlanningWeekWeights() @as defaultPlanningWeights
        {ScalarSetting {
            SettingKey "planning_week_weights"
            SettingValue defaultPlanningWeights
        }}
        {ScalarSetting {SettingKey? "planning_week_weights"}} @as [PlanningWeightsSetting]
    }
    helper.resolvePlanningWeekWeights(PlanningWeightsSetting) @as planningWeekWeights

This is verbose and not declarative. Proposing the following (syntactic sugar over @catch {not_found ...}):

    {ScalarSetting {SettingKey? "planning_week_weights"}} 
    @empty {ScalarSetting {SettingKey? "default"}} 
    @as [PlanningWeightsSetting]

The @empty pattern gets evaluated when the query returns an empty result.