Set @timestamp default value and ignore other empty values.

Prevent PHP from throwing unwanted NOTICEs whenever any of the following `$record` fields don't exist:

* `channel`
* `context`
* `datetime`
* `extra`
* `level`
* `level_name`
* `message`

Also, if `$record['datetime']` is empty, set `@timestamp` to the current ISO8601 timestamp by calling `gmdate('c')`. (according to the [logstash source](https://github.com/elasticsearch/logstash/blob/master/lib/logstash/event.rb), from the settable fields, only `@timestamp` is required)

Works with both V0 and V1 formats and all tests pass after these changes.