fix: service register url's group and version key with "" value.

[LaurenceLiZhixin]

As for java client, if user set 'version' to '1.0.0" and doesn't set 'group' key. the reference url's param contains version=1.0.0 and doesn't contain 'group' key.
But golang server regster 'group' key with "" value even if user didn't configure service's 'group' field in config file.
That's a bug. I open this pr is to let url to be register, doesn't contain group or value key, if user didn't set.

[codecov-commenter]

Codecov Report

Merging #1612 (5e2b72f) into 3.0 (1c92dde) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              3.0    #1612      +/-   ##
==========================================
- Coverage   40.94%   40.91%   -0.03%     
==========================================
  Files         251      251              
  Lines       14403    14405       +2     
==========================================
- Hits         5897     5894       -3     
- Misses       7826     7830       +4     
- Partials      680      681       +1     

Impacted Files	Coverage Δ
config/service_config.go	10.90% <0.00%> (-0.08%)	⬇️
filter/metrics/filter.go	85.00% <0.00%> (-15.00%)	⬇️

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