FRR: fix NPE in routemaps for non-bgp routes

[progwriter]

This is unlikely to correctly capture FRR semantics until we overhaul routing policy, but at least will prevent crashes.

[batfish-bot]

This change is 

[dhalperi]

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

[progwriter]

Reviewed 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #6453 (286069f) into master (42af8f4) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #6453      +/-   ##
============================================
- Coverage     73.18%   73.17%   -0.01%     
+ Complexity    35395    35393       -2     
============================================
  Files          2820     2820              
  Lines        143228   143228              
  Branches      17291    17292       +1     
============================================
- Hits         104816   104808       -8     
- Misses        30117    30123       +6     
- Partials       8295     8297       +2     

Impacted Files	Coverage Δ	Complexity Δ
...datamodel/routing_policy/statement/Statements.java	80.48% <100.00%> (ø)	2.00 <0.00> (ø)
...g/batfish/datamodel/flow/IncomingSessionScope.java	84.61% <0.00%> (-15.39%)	7.00% <0.00%> (-1.00%)
...rg/batfish/identifiers/StorageBasedIdResolver.java	85.29% <0.00%> (-5.89%)	22.00% <0.00%> (ø%)
...main/java/org/batfish/datamodel/acl/AclTracer.java	60.37% <0.00%> (-1.26%)	43.00% <0.00%> (-1.00%)
...ain/java/org/batfish/storage/FileBasedStorage.java	86.19% <0.00%> (-0.28%)	249.00% <0.00%> (ø%)