This change was suggested by issue #2316. There was a problem reading Xlsx comments which appeared with release 18.0 but which was already fixed in master. So no source change was needed to fix the issue, but I thought we should at least add the test case to our unit tests.
In developing that case, I discovered that, although comment text was read correctly, there was a problem with comment author. In fact, there were two problems. One was new, with the namespacing changes - as in several other cases, the namespaced attribute `authorId` needed some special handling. However, another problem was much older - the code was checking `!empty($comment['authorId'])`, eliminating consideration of authorId=0, and should instead have been checking `isset`. Both problems are now fixed, and tested.
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