dialect/sql/sqlgraph: fix CreateBulk when Fields in BatchCreateSpec is empty

[t-coron]

Fixed #3944 Issue in CreateBulk()

CraeteBulk() always generates wrong query when Fields in batchCreateSpec.Nodes is empty

query=INSERT INTO `groups` VALUES () args=[]

This query is generated by:

ent/dialect/sql/sqlgraph/graph.go

Line 1508 in d4560fa

insert := c.builder.Insert(c.Nodes[0].Table).Schema(c.Nodes[0].Schema).Default().Columns(sorted...)

If defaults is true and len(sorted) (i.e. len(columns)) is zero, insert builder exec writeDefault(),

ent/dialect/sql/builder.go

Lines 963 to 965 in d4560fa

	if i.defaults && len(i.columns) == 0 {
	i.writeDefault(&b)
	} else {

ent/dialect/sql/builder.go

Lines 982 to 989 in d4560fa

	func (i *InsertBuilder) writeDefault(b *Builder) {
	switch i.Dialect() {
	case dialect.MySQL:
	b.WriteString("VALUES ()")
	case dialect.SQLite, dialect.Postgres:
	b.WriteString("DEFAULT VALUES")
	}
	}

With this PR, we try to avoid execution of writeDefault.
If we want to insert 3 records through CreateBulk(), the query is:

query=INSERT INTO `groups` (`id`) VALUES (NULL), (NULL), (NULL) args=[]

[t-coron]

Umm, this might be better.

// if node.ID != nil && (node.ID.Value != nil || len(node.Fields) == 0) {
if node.ID != nil {