CumulusNclu: be robust to bad interface neighbor

[dhalperi]

By creating a dummy neighbor, we prevent crashes handling the rest of the line.

[batfish-bot]

This change is 

[progwriter]

Reviewed 3 of 3 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #6471 (e4582ea) into master (2653a3e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #6471      +/-   ##
============================================
- Coverage     73.29%   73.28%   -0.01%     
+ Complexity    35459    35454       -5     
============================================
  Files          2822     2822              
  Lines        143314   143321       +7     
  Branches      17299    17299              
============================================
- Hits         105038   105037       -1     
- Misses        29956    29960       +4     
- Partials       8320     8324       +4     

Impacted Files	Coverage Δ	Complexity Δ
.../cumulus_nclu/CumulusNcluConfigurationBuilder.java	86.84% <100.00%> (+0.66%)	253.00 <0.00> (+1.00)
...col/src/main/java/org/batfish/role/InferRoles.java	89.54% <0.00%> (-1.37%)	50.00% <0.00%> (-1.00%)
...src/main/java/org/batfish/coordinator/WorkMgr.java	75.42% <0.00%> (-0.55%)	245.00% <0.00%> (-3.00%)
...a/org/batfish/representation/aws/LoadBalancer.java	82.18% <0.00%> (-0.32%)	70.00% <0.00%> (-1.00%)
...ish/representation/cumulus/CumulusConversions.java	91.30% <0.00%> (-0.05%)	194.00% <0.00%> (-2.00%)
...sh/grammar/arista/AristaControlPlaneExtractor.java	35.01% <0.00%> (+0.02%)	613.00% <0.00%> (+1.00%)
...ish/representation/arista/AristaConfiguration.java	58.28% <0.00%> (+0.07%)	178.00% <0.00%> (+2.00%)