[crac] RFR: Fixing crash on restore when user name is not set

[Roman Marchenko]

This change fixes a crash occuring during restore in a container when user name is not set.

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Commit messages:
 - Fixing crash on restore when user name is not set

Changes: https://git.openjdk.org/crac/pull/129/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=129&range=00
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/crac/pull/129.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/129/head:pull/129

PR: https://git.openjdk.org/crac/pull/129