Arista: handle undefined references in dynamic source nat

[dhalperi]

Fix a crash.

[batfish-bot]

This change is 

[dhalperi]

Reviewable status: 0 of 4 files reviewed, all discussions resolved (waiting on @corinaminer and @sfraint)

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projects/batfish/src/main/java/org/batfish/representation/arista/AristaConfiguration.java line 1378 at r1 (raw file):

      if (!_extendedAccessLists.containsKey(nat.getNatAclName())) {
        // https://www.arista.com/en/um-eos/eos-data-plane-security#xx1143262
        // Commands referencing nonexistent ACLs are accepted by the CLI but not installed in

FYI copied from a few lines up (static nat)

[codecov]

Codecov Report

Merging #8310 (f3f2878) into master (c674c38) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #8310      +/-   ##
============================================
- Coverage     74.61%   74.60%   -0.01%     
+ Complexity    43897    43895       -2     
============================================
  Files          3410     3410              
  Lines        169889   169895       +6     
  Branches      20310    20311       +1     
============================================
- Hits         126765   126757       -8     
- Misses        33531    33544      +13     
- Partials       9593     9594       +1     

Impacted Files	Coverage Δ
...ish/representation/arista/AristaConfiguration.java	64.91% <100.00%> (+0.12%)	⬆️
.../representation/arista/AristaDynamicSourceNat.java	86.66% <100.00%> (+0.95%)	⬆️
...src/main/java/org/batfish/coordinator/PoolMgr.java	59.52% <0.00%> (-1.20%)	⬇️
...ain/java/org/batfish/storage/FileBasedStorage.java	85.47% <0.00%> (-0.86%)	⬇️
...batfish/src/main/java/org/batfish/main/Driver.java	49.18% <0.00%> (-0.55%)	⬇️
...java/org/batfish/dataplane/ibdp/VirtualRouter.java	87.56% <0.00%> (-0.25%)	⬇️
...src/main/java/org/batfish/coordinator/WorkMgr.java	75.81% <0.00%> (-0.16%)	⬇️
.../org/batfish/dataplane/ibdp/BgpRoutingProcess.java	83.67% <0.00%> (-0.09%)	⬇️

[sfraint]

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @corinaminer)