Ignore a nil Or in Where, matching nil predicates

[AshSgDe29071999]

Fixes #382

Where(nil) is documented as a no-op, but a helper typed as sq.Or that returns nil is a typed-nil interface. That is not == nil, so it was compiled as WHERE (1=0) and inverted optional filters.

func getFilter() sq.Or { return nil }
Select("*").From("users").Where(getFilter())
// before: SELECT * FROM users WHERE (1=0)
// after:  SELECT * FROM users

An empty Or{} is unchanged (1=0). The same ignore is applied on Update and Delete Where for consistency.