fix: support xds adsz empty metadata

[LaurenceLiZhixin]

What this PR does:
Now, if istiod adsz API returns empty registry result, it would panic.
This pr supports empty dubbogo metadata, and add uts.

Which issue(s) this PR fixes:
Fixes #

You should pay attention to items below to ensure your pr passes our ci test
We do not merge pr with ci tests failed

• All ut passed (run 'go test ./...' in project root)
• After go-fmt ed , run 'go fmt project' using goland.
• Golangci-lint passed, run 'sudo golangci-lint run' in project root.
• After import formatted, (using imports-formatter to run 'imports-formatter .' in project root, to format your import blocks, mentioned in CONTRIBUTING.md above)
• Your new-created file needs to have apache license at the top, like other existed file does.
• All integration test passed. You can run integration test locally (with docker env). Clone our dubbo-go-samples project and replace the go.mod to your dubbo-go, and run 'sudo sh start_integration_test.sh' at root of samples project root. (M1 Slice is not Support)

[codecov-commenter]

Codecov Report

Merging #1813 (5ecec59) into 3.0 (d8b2baa) will decrease coverage by 0.09%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##              3.0    #1813      +/-   ##
==========================================
- Coverage   46.88%   46.78%   -0.10%     
==========================================
  Files         298      298              
  Lines       17148    17159      +11     
==========================================
- Hits         8039     8027      -12     
- Misses       8264     8285      +21     
- Partials      845      847       +2     

Impacted Files	Coverage Δ
remoting/xds/mapping/debug.go	68.42% <50.00%> (-31.58%)	⬇️
metadata/report/delegate/delegate_report.go	26.49% <0.00%> (-7.95%)	⬇️
...tocol/rest/server/server_impl/go_restful_server.go	41.37% <0.00%> (-3.45%)	⬇️
metrics/prometheus/reporter.go	26.85% <0.00%> (-1.72%)	⬇️

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[binbin0325]

LGTM