[MEI encoding] - File sometimes contains an all-zero zone that isn't linked to anything in the file

An example is folio 232v. The second zone from the top has all of its coordinates `="0"` and its ID doesn't refer to anything else in the file.

Folio 232v:
[232v with staging test neume on two staves.mei 2.zip](https://github.com/DDMAL/Rodan/files/13587865/232v.with.staging.test.neume.on.two.staves.mei.2.zip)

This is very similar to https://github.com/DDMAL/Rodan/issues/643, but in that case the all-zero zone was always associated to the first syllable's bounding box, so the ID referred to an element in the file. In fact, all the other all-zero zones I can find are associated with a syllable's bounding box. Is it possible that in the case of 232v that syllable was empty, so MEI_encoding deleted it without deleting its bounding box zone?