Model.read creates empty node tables for each node type

[visr]

Preferably these node tables are kept None, such that it is clear that they are not present, e.g. in _nodes() from #1270. With #1279 we now work around this by not only checking node.df is None but additionally attr.node.df.empty.

In #1276 I came across this issue and try to adapt the filter function, which caused several other issues.

With the workaround the issue itself is not so pressing, though it is worth seeing if we fully understand and perhaps can simplify some of the validation logic causing the unwanted side effects seen in #1276.