Do not start an IMAP loop for sent folder if watching sentbox is enabled but no sentbox is configured

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Currently in the Python test test_mvbox_sentbox_threads the option is enabled, but because CI server nine.testrun.org does not have a Sent folder, IMAP loop is started, opens a connection and goes to fake idle. We should not open a connection at all in this case.

This is why the test is marked as xfail in https://github.com/deltachat/deltachat-core-rust/pull/5104