Avoid creating log file if nothing flushed

[dentiny]

I'm using rotating logger for error message, one thing I found is, even if there's no error message, spdlog still creates error log file.
Code pointer here:

spdlog/include/spdlog/sinks/rotating_file_sink-inl.h

Line 44 in ae1de0d

file_helper_.open(calc_filename(base_filename_, 0));

I'm wondering if we could avoid creating file if nothing written to spdlog, or at least we provide such option for lazy file creation?

Two benefits:

• Cleaner user experience, it's weird to see an empty log;
• Performance, we will not have any syscalls if nothing written.

If owners feel fine for the proposal, I'm willing to create PR and implement the feature.