Avoid panics when DBName is not set

provider/aws: The DBName in RDS instance is optional and when
not set, the Read function return null.

builtin/providers/aws/resource_aws_db_instance.go

@@ -304,7 +304,11 @@ func resourceAwsDbInstanceRead(d *schema.ResourceData, meta interface{}) error {
 		return nil
 	}
 
+	if v.DBName != nil {
 		d.Set("name", *v.DBName)
+	} else {
+		d.Set("name", "")
+	}
 	d.Set("username", *v.MasterUsername)
 	d.Set("engine", *v.Engine)
 	d.Set("engine_version", *v.EngineVersion)