Backport(v1.19): parser_csv: skip empty or unparseable lines (#5355)

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Which issue(s) this PR fixes:
Backport #5355
Fixes #

What this PR does / why we need it:
This PR fixes an issue in parser_csv where processing an empty line causes a TypeError.

When parser_csv receives an empty line, CSV.parse_line returns nil.
Currently, passing this nil value to Array#zip throws a TypeError: wrong argument type NilClass (must respond to :each).

Reproduce

config:

<source>
  @type tail
  path "#{File.expand_path '~/tmp/fluentd/test.csv'}"
  tag test.csv
  read_from_head true
  <parse>
    @type csv
    keys k1,k2,k3
  </parse>
</source>

<match test.csv>
  @type stdout
</match>

Prepare input data:

$ echo "A,B,C"  > ~/tmp/fluentd/test.csv
$ echo ""      >> ~/tmp/fluentd/test.csv
$ echo "D,E,F" >> ~/tmp/fluentd/test.csv

Result of before changing:

2026-05-09 17:23:14 +0900 [info]: #0 following tail of /home/watson/tmp/fluentd/test.csv
2026-05-09 17:23:14 +0900 [warn]: #0 invalid line found 
2026-05-09 17:23:14 +0900 [warn]: #0 this parameter is highly recommended to save the position to resume tailing.
2026-05-09 17:23:14 +0900 [info]: #0 starting fluentd worker pid=41671 ppid=41646 worker=0
2026-05-09 17:23:14 +0900 [info]: #0 following tail of /home/watson/tmp/fluentd/test.csv
2026-05-09 17:23:14 +0900 [warn]: #0 invalid line found file="/home/watson/tmp/fluentd/test.csv" line="" error="wrong argument type NilClass (must respond to :each)"
2026-05-09 17:23:14.692710744 +0900 test.csv: {"k1":"A","k2":"B","k3":"C"}
2026-05-09 17:23:14.692801343 +0900 test.csv: {"k1":"D","k2":"E","k3":"F"}
2026-05-09 17:23:14 +0900 [info]: #0 fluentd worker is now running worker=0

Docs Changes:
N/A

Release Note:

• parser_csv: skip empty or unparseable lines