[Issue 18857] [std.experimental.logger] use NullLogger as the default

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18857
Issue 18857 depends on issue 22532, which changed state.

Issue 22532 Summary: std.experimental.logger Change default log level to LogLevel.warning, or LogLevel.off
https://issues.dlang.org/show_bug.cgi?id=22532

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

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