New timestamp default option changed the default behavior for missing paths

[Mpdreamz]

PR #7036 introduced a new default option to the _timestamp mapping.

The main goal was to combat the case when you have a _timestamp mapping with path set but index a document without the given path present.

e.g you have this mapping:

"_timestamp" : {
    "enabled" : "yes",
    "path" : "post_date"
}

and present this document:

{
  "foo": "bar"
}

pre 1.3 the indexation would fail, using 1.4 we default to using now() for _timestamp when path is missing.
To reintroduce the 1.3 behavior you now have to pass default: null giving null the special meaning of mandatory. I can not think of anywhere else where we give null a special meaning.

I personally feel that when you map _timestamp with _path the default result when you index a document without that path should be an exception (as it was before 1.4).

If you know your data may not contain the value you should proactively configure for this situation setting a bool ignore_missing that falls back to the behavior had path not been specified in the mapping in the first place.

1.3 behaviour with a new ignore_missing bool configuration:

_timestamp config	Result.
enabled	now ()
using path	value of path or 💥
ignore_missing	value of path or now()

1.4 handling of _timestamp

_timestamp config	Result.
enabled	now()
using path	value or now ()
using default	now(), <fixed date> or 💥 if set to null

Out of these two the 1.3 way of handling _timestamp mappings makes the most sense to me.

[clintongormley]

I agree - the default if a path is specified should be null