New timestamp default option changed the default behavior for missing paths

PR #7036 changed the behavior for timestamp when provided as a parameter or within the document using `path` attribute.

This PR now considers that:

* when using timestamp as a parameter, we use a default value of `now`. Which means that if no timestamp is provided, the current time is used when the index operation is performed.
* when getting the timestamp from `path`, we use a default value of `null`. Which means that if no value is provided within the document, indexation will fail.

If users want to set the default value for `timestamp`, they can explicitly set one or set `"default":  "now"`.

Closes #8882.