Border cases not handled

[laurentbartholdi]

Lots of border cases are not handled properly:

julia> using SymbolicWedderburn, PermutationGroups, StarAlgebras

julia> tbl = SymbolicWedderburn.CharacterTable(Rational{Int}, PermutationGroups.PermGroup(perm"()")) # freezes

julia> tbl = SymbolicWedderburn.CharacterTable(Rational{Int}, PermutationGroups.PermGroup()) # errors

julia> tbl = SymbolicWedderburn.CharacterTable(Rational{Int}, PermutationGroups.PermGroup(perm"(1,2)"))

julia> χ = SymbolicWedderburn.irreducible_characters(tbl)[1]

julia> zero(χ) # missing method

julia> SymbolicWedderburn.dot(0χ,0χ) # errors

[kalmarek]

I believe all of those have been fixed; Please reopen if necessary.