feature: convert_...() : warning when importing null or negative segments

[florianschiel]

Feature Request:
Currently it is possible to import a DB with zero or negative length segments via the convert_...() functions (tested on convert_legacyEmuDB()). Although this works within the emuDB concept, the EmuLabeller will not cope with that (and their are other problems when working on these data).
It would be nice,

• to get a warning, when such segments are imported in all convert_...()
• to have an option removeNullSegments=T that automatically prevents such segments to be imported, so that the resulting emuDB can be manipulated by the EmuLabeller (and other functions that require consecutive, non-zero segments).