use level config in main section when subsection not set level (#15176)

use level config in main section when subsection not set level (#15176)

in previouse if a log subsetcion not set level
it will use ``info`` as default value.

this pr will make default value (``[log] -> LEVEL``) useable.

example config:
```INI
[log]
MODE = console
LEVEL = Trace

[log.console]
LEVEL =
STDERR = false
```

previous result:
```JSON
// console:
{
  "level": "info",
  ...................
}
```

after change:

```JSON
// console:
{
  "level": "track",
  ...................
}
```

Signed-off-by: a1012112796 <1012112796@qq.com>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>