Fix proxy panic when balancer has no targets

[shblue21]

Fixes #2976

This fixes a panic when a proxy balancer returns no target.

Built-in balancers can return nil when the target list is empty, for example after RemoveTarget removes the last target. ProxyWithConfig now returns 502 Bad Gateway through the configured error handler instead of passing the nil target to proxyHTTP or proxyRaw.

I also added a test for a custom balancer returning a target with a nil URL.

Tests pass with go test ./... -count=1.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.17%. Comparing base (01b45be) to head (7b5690a).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2977      +/-   ##
==========================================
- Coverage   93.17%   93.17%   -0.01%     
==========================================
  Files          43       43              
  Lines        4501     4499       -2     
==========================================
- Hits         4194     4192       -2     
  Misses        189      189              
  Partials      118      118              

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[aldas]

LGTM

[aldas]

Thanks