[Configuration] Allow specifying an explicit NULL imply default values in the configuration

// IN THE BASE
NULL = use default
MISSING = use default
{ empty } = use default

// IN THE OVERRIDE
NULL = use default
MISSING = use base
{ empty } = use base

// one
"user": { "first": "jerry", "last": "nixon" }
"user": { empty }
"user": { "first": "jerry", "last": "nixon" }

// IMPOSSIBLE
// one-b
"user": { "first": "jerry", "last": "nixon" }
"user": NULL
"user": { "first": DEFAULT, "last": DEFAULT }

// IMPOSSIBLE
// two
"user": { "first": "jerry", "last": "nixon" }
"user": { "first": NULL, "last": NULL }
"user": { "first": DEFAULT, "last": DEFAULT }

// three
"user": { "first": "jerry", "last": "nixon" }
"user": { "first": "jerry" }
"user": { "first": "jerry", "last": "nixon" }

// IMPOSSIBLE
// four
"user": { "first": "jerry", "last": "nixon" }
"user": { "first": "jerry", "last": NULL }
"user": { "first": "jerry", "last": DEFAULT }

// five
"user": { "first": "jerry", "last": "nixon" }
"user": MISSING (not specified in override)
"user": { "first": "jerry", "last": "nixon" }