Cumulus: fix conversion when only L2 VNIs are present

[progwriter]

By creating extra dummy BGP processes during conversion.
Fixes a dataplane crash. Follow-up to #5443

[batfish-bot]

This change is 

[ratulm]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #5454 into master will decrease coverage by <.01%.
The diff coverage is 75%.

@@             Coverage Diff              @@
##             master    #5454      +/-   ##
============================================
- Coverage     73.45%   73.45%   -0.01%     
+ Complexity    32079    32077       -2     
============================================
  Files          2620     2620              
  Lines        129029   129041      +12     
  Branches      15538    15538              
============================================
+ Hits          94780    94781       +1     
- Misses        26710    26718       +8     
- Partials       7539     7542       +3

Impacted Files	Coverage Δ	Complexity Δ
...ish/representation/cumulus/CumulusConversions.java	89.59% <75%> (-0.02%)	162 <4> (ø)
...org/batfish/datamodel/flow/BidirectionalTrace.java	81.81% <0%> (-2.28%)	14% <0%> (-1%)
...src/main/java/org/batfish/coordinator/PoolMgr.java	64.04% <0%> (-1.13%)	15% <0%> (-1%)
...ar/cumulus_frr/CumulusFrrConfigurationBuilder.java	76.52% <0%> (-0.38%)	126% <0%> (+3%)
...batfish/src/main/java/org/batfish/main/Driver.java	38.07% <0%> (-0.26%)	28% <0%> (ø)
...tfish/representation/cisco/CiscoConfiguration.java	85.49% <0%> (-0.13%)	524% <0%> (-1%)
...tion/cumulus/CumulusConcatenatedConfiguration.java	67.32% <0%> (+0.08%)	75% <0%> (ø)	⬇️