fix spansql: CREATE PROTO BUNDLE SQL with 0 types

Fix a bug in CreateProtoBundle.SQL() which unintentionally generated the
DDL when there were no types listed:
```
CREATE PROTO BUNDLE (``)
```

spanner/spansql/sql.go

@@ -99,8 +99,12 @@ func (ci CreateIndex) SQL() string {
 }
 
 func (cp CreateProtoBundle) SQL() string {
+	typeList := ""
+	if len(cp.Types) > 0 {
+		typeList = "`" + strings.Join(cp.Types, "`, `") + "`"
+	}
 	// Backtick-quote all the types so we don't need to check for SQL keywords
-	return "CREATE PROTO BUNDLE (`" + strings.Join(cp.Types, "`, `") + "`)"
+	return "CREATE PROTO BUNDLE (" + typeList + ")"
 }
 
 func (cv CreateView) SQL() string {

spanner/spansql/sql_test.go

@@ -944,6 +944,14 @@ func TestSQL(t *testing.T) {
 			"CREATE PROTO BUNDLE (`a.b.c`, `b.d.e`)",
 			reparseDDL,
 		},
+		{
+			&CreateProtoBundle{
+				Types:    []string(nil),
+				Position: line(1),
+			},
+			"CREATE PROTO BUNDLE ()",
+			reparseDDL,
+		},
 		{
 			&CreateProtoBundle{
 				Types:    []string{"a"},