Fluent Bit: ログがすべて(null)になる

## 概要
Fluent Bit v1.9.3でFile Outputを使うとき、デフォルトの`out_file`フォーマットを使うとログがすべて`(null)`になってしまう。
別のフォーマットを使うとこの問題は起きなかった。

## 例
例えば、次のような設定でFluent Bitを実行すると

```fluent-bit.conf
[INPUT]
    name            cpu
    tag             cpu
    interval_sec    1

[INPUT]
    Name            systemd
    Path            /var/log/journal/
    Tag             *
    Systemd_Filter  _SYSTEMD_UNIT=fluent-bit.service

[OUTPUT]
    Name            file
    Match           *
    Path            /var/log/fluent-bit
```

タイムスタンプと`(null)`とだけ書かれたファイルが生成された。

```:/var/log/fluent-bit/cpu
cpu: [8441803988342570310.000000000, (null)]
cpu: [8441803988226873342.000000000, (null)]
cpu: [8441803988226903519.000000000, (null)]
```

```:/var/log/fluent-bit/fluent-bit.service
fluent-bit.service: [8430896833085458256.000000000, (null)]
fluent-bit.service: [8431160715876124496.000000000, (null)]
fluent-bit.service: [8431160715876124496.000000000, (null)]
```

別のフォーマット、例えば`plain`を使うように設定すると

```fluent-bit.conf
[INPUT]
    name            cpu
    tag             cpu
    interval_sec    1

[INPUT]
    Name            systemd
    Path            /var/log/journal/
    Tag             *
    Systemd_Filter  _SYSTEMD_UNIT=fluent-bit.service

[OUTPUT]
    Name            file
    Match           *
    Path            /var/log/fluent-bit
    Format          plain
```

無事ログが出力されるようになった。

```json:/var/log/fluent-bit/cpu
{"cpu_p":6.9,"user_p":5.475,"system_p":1.425,"cpu0.p_cpu":2.9,"cpu0.p_user":0.9,"cpu0.p_system":2.0,"cpu1.p_cpu":20.4,"cpu1.p_user":18.7,"cpu1.p_system":1.7,"cpu2.p_cpu":2.8,"cpu2.p_user":1.2,"cpu2.p_system":1.6,"cpu3.p_cpu":1.5,"cpu3.p_user":1.1,"cpu3.p_system":0.4}
{"cpu_p":0.075,"user_p":0.0,"system_p":0.075,"cpu0.p_cpu":0.1,"cpu0.p_user":0.0,"cpu0.p_system":0.1,"cpu1.p_cpu":0.0,"cpu1.p_user":0.0,"cpu1.p_system":0.0,"cpu2.p_cpu":0.0,"cpu2.p_user":0.0,"cpu2.p_system":0.0,"cpu3.p_cpu":0.2,"cpu3.p_user":0.0,"cpu3.p_system":0.2}
{"cpu_p":0.225,"user_p":0.1,"system_p":0.125,"cpu0.p_cpu":0.4,"cpu0.p_user":0.1,"cpu0.p_system":0.3,"cpu1.p_cpu":0.1,"cpu1.p_user":0.1,"cpu1.p_system":0.0,"cpu2.p_cpu":0.1,"cpu2.p_user":0.0,"cpu2.p_system":0.1,"cpu3.p_cpu":0.3,"cpu3.p_user":0.2,"cpu3.p_system":0.1}
```

```json:/var/log/fluent-bit/fluent-bit.service
{"_COMM":"fluent-bit","_EXE":"/opt/fluent-bit/bin/fluent-bit","_CMDLINE":"/opt/fluent-bit/bin/fluent-bit -c //etc/fluent-bit/fluent-bit.conf","_SYSTEMD\_CGROUP":"/system.slice/fluent-bit.service","_SYSTEMD_UNIT":"fluent-bit.service","MESSAGE":"[2022/05/14 03:10:23] [ info] [storage] version=1.2.0, type=memory-only, sync=normal, checksum=disabled, max_chunks_up=128"}
{"_COMM":"fluent-bit","_EXE":"/opt/fluent-bit/bin/fluent-bit","_CMDLINE":"/opt/fluent-bit/bin/fluent-bit -c //etc/fluent-bit/fluent-bit.conf","_SYSTEMD_CGROUP":"/system.slice/fluent-bit.service","_SYSTEMD_UNIT":"fluent-bit.service","MESSAGE":"[2022/05/14 03:10:23] [ info] [cmetrics] version=0.3.1"}
{"_COMM":"fluent-bit","_EXE":"/opt/fluent-bit/bin/fluent-bit","_CMDLINE":"/opt/fluent-bit/bin/fluent-bit -c //etc/fluent-bit/fluent-bit.conf","_SYSTEMD_CGROUP":"/system.slice/fluent-bit.service","_SYSTEMD_UNIT":"fluent-bit.service","MESSAGE":"[2022/05/14 03:10:23] [ info] [input:systemd:systemd.0] seek_cursor=s=f707fe14e7dd47f2b211b7fea1727297;i=268... OK"}
```

## 参考
- [systemd input plugin spamming with null records](https://github.com/fluent/fluent-bit/issues/1404)
- [Null log lines at high rates](https://github.com/fluent/fluent-bit/issues/5214)