Fix native OTel logs collection where 0 length logs cause errors

[jvoravong]

Fix native OTel logs collection where 0 length log lines cause errors after the 0.29.0 opentelemetry-logs-library changes in v0.49.0 of the collector.

Testing:
Verified this is only an issue with v0.49.0 of the collector and after.

Example:
An application logs an error stack trace like such:

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/local/lib/python3.8/site-packages/requests/adapters.py", line 439, in send
...

The collector will throw an error like such:

2022-05-03T20:44:50.359Z error helper/transformer.go:110 Failed to process entry {"kind": "receiver", "name": "filelog", "operator_id": "parser-containerd", "operator_type": "regex_parser", "error": "regex pattern does not match", "action": "send", "entry": {"observed_timestamp":"2022-05-03T20:44:50.359464261Z","timestamp":"0001-01-01T00:00:00Z","body":"2022-05-03T20:44:36.873466616Z stderr F","attributes":{"log.file.path":"/var/log/pods/xxxx/server/0.log"},"severity":0,"scope_name":""}}

If these changes are merged, I plan on submitting a PR for the opentelemetry-collector-contrib example that is related.