initData
Provides the initial values for the form's controls.
Every key must point at a control, and every value must fit that control's type - otherwise the engine throws InitDataException with that path before the form is drawn.
Return
map control -> value with default or computed values. null means loading failed and the method has already shown why (usually through loadData) - the form is then left without controls. Like false from an action, null carries the fact of failure, not its content.
Parameters
payload
additional data for the form (an empty map when there is none)