Need some distinctive internal default for task If parameter

[nightroman]

The tweak is needed for #136 in order to be able to recognize conditional tasks in Show-BuildGraph.ps1. Currently, the internal default value of the parameter If is $true. It is very natural indeed. But we cannot distinguish conditional and not conditional tasks using this value, it might be both the default and the result of evaluated If. We need some more distinctive equivalent of true, any value will do. This approach is hacky but reasonable for the tools like Show-BuildGraph.ps1.