Replace empty set symbol for empty values

[arildm]

The symbol ∅ (empty set) has been gradually introduced to denote an empty value (word picture: ed133f9, statistics: 1335c6c, trend table: 50dd2d8, sidebar: fb346b4)

However, it can be confusing to some users, as it suggests that the type of the value is a set or a list. (Some attributes are indeed list-typed, but that's not what the symbol is used for.)

One suggestion is to replace it with a dash, like we have done previously. Dashes are more frequent in data, though, and are likely to be confused with an actual data value.