xds/googledirectpath: Check if ipv6 address is non empty

[zasweq]

This PR checks if the returned ipv6 address is non empty when queried in the google direct path resolver.

RELEASE NOTES: N/A

[codecov]

Codecov Report

Merging #6959 (655a331) into master (cd69b5d) will decrease coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is 65.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6959      +/-   ##
==========================================
- Coverage   83.74%   83.72%   -0.02%     
==========================================
  Files         287      287              
  Lines       30929    30915      -14     
==========================================
- Hits        25900    25885      -15     
- Misses       3971     3973       +2     
+ Partials     1058     1057       -1     

Files	Coverage Δ
internal/grpcrand/grpcrand_go1.21.go	77.77% <77.77%> (ø)
xds/googledirectpath/utils.go	54.90% <20.00%> (-4.68%)	⬇️

... and 16 files with indirect coverage changes