OC trace fix

[csanadtelbisz]

Minor fix of assembling a trace in OC checker when an empty XCFA edges is encountered.
(Unrelated issue to #457, but we might wait until we resolve that one too.)

[github-actions]

Benchexec test report for a selection of SV-Benchmarks (correct / incorrect / all):

Rundefinition	COMPLEX	EMERGENT	HORN
SV-COMP25_no-data-race	✅ (59 / 0 / 89) HTML/CSV	✅ (32 / 0 / 68) HTML/CSV	✅ (20 / 0 / 83) HTML/CSV
SV-COMP25_no-overflow	✅ (17 / 0 / 32) HTML/CSV	❓ (0 / 0 / 37) HTML/CSV	❓ (0 / 0 / 90) HTML/CSV
SV-COMP25_termination	✅ (1 / 0 / 204) HTML/CSV	✅ (1 / 0 / 202) HTML/CSV	✅ (14 / 0 / 304) HTML/CSV
SV-COMP25_unreach-call	✅ (18 / 0 / 96) HTML/CSV	✅ (15 / 0 / 32) HTML/CSV	✅ (9 / 0 / 191) HTML/CSV
SV-COMP25_valid-memcleanup	❓ (0 / 0 / 66) HTML/CSV	❓ (0 / 0 / 66) HTML/CSV	❓ (0 / 0 / 66) HTML/CSV
SV-COMP25_valid-memsafety	✅ (34 / 0 / 185) HTML/CSV	✅ (3 / 0 / 267) HTML/CSV	❓ (0 / 0 / 335) HTML/CSV