ibdp/Node: use Optional#flatMap correctly

[dhalperi]

We accidentally made an Optional<Optional<Rib>> that could be absent, or
present with a Rib, or present with an absent Rib. We meant to chain the
optionality, which is done with flatMap.

[batfish-bot]

This change is 

[codecov]

Codecov Report

Merging #7043 (196f3ef) into master (c9c89d8) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #7043      +/-   ##
============================================
- Coverage     71.92%   71.90%   -0.03%     
+ Complexity    37727    37715      -12     
============================================
  Files          3036     3036              
  Lines        154786   154786              
  Branches      18576    18576              
============================================
- Hits         111333   111302      -31     
- Misses        34571    34593      +22     
- Partials       8882     8891       +9     

Impacted Files	Coverage Δ
...src/main/java/org/batfish/dataplane/ibdp/Node.java	81.81% <100.00%> (+9.09%)	⬆️
.../datamodel/routing_policy/statement/Statement.java	72.72% <0.00%> (-9.10%)	⬇️
...src/main/java/org/batfish/coordinator/PoolMgr.java	54.76% <0.00%> (-5.96%)	⬇️
...rg/batfish/identifiers/StorageBasedIdResolver.java	85.29% <0.00%> (-5.89%)	⬇️
.../org/batfish/dataplane/rib/RouteAdvertisement.java	83.67% <0.00%> (-4.09%)	⬇️
...ain/java/org/batfish/symbolic/IngressLocation.java	65.78% <0.00%> (-2.64%)	⬇️
...col/src/main/java/org/batfish/role/InferRoles.java	91.36% <0.00%> (-1.37%)	⬇️
...main/java/org/batfish/datamodel/acl/AclTracer.java	62.42% <0.00%> (-1.28%)	⬇️
...ain/java/org/batfish/storage/FileBasedStorage.java	84.50% <0.00%> (-0.93%)	⬇️
...ain/java/org/batfish/coordinator/WorkQueueMgr.java	70.93% <0.00%> (-0.59%)	⬇️
... and 2 more

[corinaminer]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @dhalperi)