RateLimiterConfig should have default values

[Tanky-Zhang]

Is your feature request related to a problem? Please describe.
When I use the limiter, I find that if the limiter is not initialized or has been deleted，now "NumberFormatException" occurs if I invoke "limiter.getConfig().getRate()".
example:

Describe the solution you'd like
After debuging, it was found that there is no nullifying process for map in the source code. I think there should be a better way to handle it, such as giving the default value or throwing an exception where the limiter does not exist, so I modified it and submitted pr.
#4608

[cy]

I'm also running into this issue - maybe instead of throwing or returning a default value it could simply return null?

[Tanky-Zhang]

I'm also running into this issue - maybe instead of throwing or returning a default value it could simply return null?

yes,"return null" is also suitable,maybe more suitable,but it seems that maintiners don‘t think this is a problem.

[mrniko]

merged