add null value, fix null expr within function

Author: fupelaqu

sql/src/main/scala/app/softnetwork/elastic/sql/SQLFunction.scala

@@ -6,7 +6,7 @@ import scala.util.matching.Regex
 sealed trait SQLFunction extends SQLRegex {
   def toSQL(base: String): String = if (base.nonEmpty) s"$sql($base)" else sql
   def applyType(in: SQLType): SQLType = out
-  var expr: SQLToken = _
+  var expr: SQLToken = SQLNull
   def applyTo(expr: SQLToken): Unit = {
     this.expr = expr
   }

sql/src/main/scala/app/softnetwork/elastic/sql/package.scala

@@ -80,6 +80,13 @@ package object sql {
     override def nullable: Boolean = false
   }
 
+  case object SQLNull extends SQLValue[Null](null) {
+    override def sql: String = "null"
+    override def painless: String = "null"
+    override def nullable: Boolean = true
+    override def out: SQLType = SQLTypes.Null
+  }
+
   case class SQLBoolean(override val value: Boolean) extends SQLValue[Boolean](value) {
     override def sql: String = value.toString
     override def out: SQLType = SQLTypes.Boolean