Index process fails when having `_timestamp` enabled and `path` option is set.
It fails with a `TimestampParsingException[failed to parse timestamp [null]]` message.
Reproduction:
```
DELETE test
PUT test
{
"mappings": {
"test": {
"_timestamp" : {
"enabled" : "yes",
"path" : "post_date"
}
}
}
}
PUT test/test/1
{
"foo": "bar"
}
```
You can define a default value for when timestamp is not provided
within the index request or in the `_source` document.
By default, the default value is `now` which means the date the document was processed by the indexing chain.
You can disable that default value by setting `default` to `null`. It means that `timestamp` is mandatory:
```
{
"tweet" : {
"_timestamp" : {
"enabled" : true,
"default" : null
}
}
}
```
If you don't provide any timestamp value, indexation will fail.
You can also set the default value to any date respecting timestamp format:
```
{
"tweet" : {
"_timestamp" : {
"enabled" : true,
"format" : "YYYY-MM-dd",
"default" : "1970-01-01"
}
}
}
```
If you don't provide any timestamp value, indexation will fail.
Closes#4718.
Closes#7036.
85eb0ea0e7