Null'ed params should pass their value to the template

[squeedee]

What happens:

in the Workload, a null param:

params:
  fred: null
  frodo: ~

is treated as "not provided" so the defaults are applied, if they exist, or an error occurs because a required param is not provided.

What should happen

null is a value, so the param should be present (and equal to nil) in the stamp context.

open questions

1. can a ytt template author discern the difference between provided and nil, and not-provided?

concerns

This could easily be considered a breaking API change.

tasks

[ ] clearly update the documentation
[ ] standard template:s must imprint "null" for a provided but null value.