Fixes the flaky mutable_object_test

[jackhumphries]

Based on a local issue I resolved, I believe this commit should fix the flaky mutable_object_test in #44396.

The header of the mutable object was not being explicitly initialized. In some cases, this caused deadlock due to a spinlock backed by garbage memory.

This commit explicitly initializes the header, which should resolve the issue.

Tested: mutable_object_test

Why are these changes needed?

See above.

Related issue number

Closes #44396.

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(