Fix extra 'AND' when len(values) == 0 ON IN.NegationBuild()

[secake]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Three reasons for this PR:

1. When DB.Not(map[string]interface{}{"name": []string{}}).Where("age = ?", 10), Generate sql SELECT * FROM ``users`` WHERE AND ``age`` = 10; , it is a bad sql.
2. In some database, like postgresql, when exec ``name`` NOT IN ('jinzhu'), the result dose not contain the row where name == NULL, so when is not in an empty array, to some extent, it can be considered that name IS NOT NULL.
3. NegationBuild() should be the inverse of Build(), but when len(values)==0, the Build() return the row where name==NULL, so NegationBuild() should not return the row where name==NULL.

User Case Description

DB.Not(map[string]interface{}{"name": []string{}}).Where("age = ?", 10).Find(&User{})`   

Thanks!

[secake]

Suggestion

In other ORM, like django, when use User.objects.filter(name__in=[]), the result dose not contain the row where name==NULL, when use User.objects.excludes(name__in=[]), the result return all rows, also contains the row where name==NULL, as follows:
select id,name from users;

id	name
1	jinzhu
2	NULL

User.objects.filter(name__in=[]).count()  // 0
User.objects.exclude(name__in=[]).count()` // 2

So I suggest the following implementation，if you think this suggestion works, I can implement it :

func (in IN) Build(builder Builder) {
	builder.WriteQuoted(in.Column)

	switch len(in.Values) {
	case 0:
		builder.WriteString(" 1=2")
	case 1:
		if _, ok := in.Values[0].([]interface{}); !ok {
			builder.WriteString(" = ")
			builder.AddVar(builder, in.Values[0])
			break
		}

		fallthrough
	default:
		builder.WriteString(" IN (")
		builder.AddVar(builder, in.Values...)
		builder.WriteByte(')')
	}
}

func (in IN) NegationBuild(builder Builder) {
	builder.WriteQuoted(in.Column)
	switch len(in.Values) {
	case 0:
		builder.WriteString(" 1=1")
	case 1:
		if _, ok := in.Values[0].([]interface{}); !ok {
			builder.WriteString(" <> ")
			builder.AddVar(builder, in.Values[0])
			break
		}

		fallthrough
	default:
		builder.WriteString(" NOT IN (")
		builder.AddVar(builder, in.Values...)
		builder.WriteByte(')')
	}
}