After 5680c38c75, postponed job APIs now
expect to be called on native threads not managed by Ruby and handles
getting a NULL execution context. However, in debug builds the change
runs into an assertion failure with GET_EC() which asserts that EC is
non-NULL. Avoid the assertion failure by passing `false` for `expect_ec`
instead as the intention is to handle when there is no EC.
Add a test from John Crepezzi and John Hawthorn to exercise this
situation.
See GH-4108
See GH-5094
[Bug #17573]
Co-authored-by: John Hawthorn <john@hawthorn.email>
Co-authored-by: John Crepezzi <john.crepezzi@gmail.com>
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