Fix ReflectionsException while scanning docstrings

[trustin]

Motivation:

DocStringsExtractor makes Reflections throw a ReflectionsException
if a resource folder (e.g. META-INF/armeria/grpc) exists but it
contains no files.

Modifications:

• Do not attempt to call Reflections.getResources() if the resource
  folder does not contain any files.

Result:

• No more ReflectionsException while scanning docstrings.
• Fixes ReflectionsException while extracting doc strings #2491

[codecov]

Codecov Report

Merging #2494 into master will increase coverage by 0.09%.
The diff coverage is 57.14%.

@@             Coverage Diff              @@
##             master    #2494      +/-   ##
============================================
+ Coverage     72.96%   73.06%   +0.09%     
- Complexity    10826    10829       +3     
============================================
  Files           948      947       -1     
  Lines         41871    41788      -83     
  Branches       5193     5175      -18     
============================================
- Hits          30553    30533      -20     
+ Misses         8670     8613      -57     
+ Partials       2648     2642       -6

Impacted Files	Coverage Δ	Complexity Δ
...necorp/armeria/server/docs/DocStringExtractor.java	64.86% <57.14%> (-1.81%)	6 <0> (ø)
.../linecorp/armeria/client/Http2ResponseDecoder.java	63.02% <0%> (-0.85%)	33% <0%> (-1%)
...armeria/server/healthcheck/HealthCheckService.java	85.2% <0%> (-0.45%)	52% <0%> (-1%)
...inecorp/armeria/server/grpc/ArmeriaServerCall.java	87.5% <0%> (ø)	86% <0%> (ø)	⬇️
...corp/armeria/internal/client/Http1ClientCodec.java
...om/linecorp/armeria/client/HttpSessionHandler.java	62.2% <0%> (+2.36%)	30% <0%> (+1%)	⬆️
...a/com/linecorp/armeria/client/HttpChannelPool.java	78.94% <0%> (+2.42%)	61% <0%> (+2%)	⬆️
...com/linecorp/armeria/client/OneEventLoopState.java	71.42% <0%> (+4.76%)	7% <0%> (+1%)	⬆️
...inecorp/armeria/client/AbstractEventLoopState.java	100% <0%> (+7.69%)	7% <0%> (+1%)	⬆️
...corp/armeria/client/DefaultEventLoopScheduler.java	89.09% <0%> (+10%)	34% <0%> (+5%)	⬆️

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[ikhoon]

Thanks!