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If you import games with ChessBase into a PGN database, then these games should not contain "Null moves", if you want to use the database with external programs. This could be e.g. PGN Reader or annotated games for your own website.
External programs cannot read these games cleanly. The reason: A "Null move" does not exist syntactically in the PGN specification. We output "--" for it. The problem is that in this case for one color two moves follow each other in the PGN notation.
A null move is a better way to illustrate certain continuations. Although a move is mandatory, no move is made for demonstration purposes.