[crac] RFR: 8351695: [CRaC] Cannot restore immediately without default restore data

[Timofei Pushkin]

The issue was not caught by the test suite because `crexec` always provides restore data even if none was set.

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Commit messages:
 - Fix keep-running with no restore data

Changes: https://git.openjdk.org/crac/pull/210/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=210&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351695
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/crac/pull/210.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/210/head:pull/210

PR: https://git.openjdk.org/crac/pull/210