lastTableName bug when first node is not "load".

When I use sdk like this:
```
test("first_node") {
val byzer = Byzer().
columns.from("table1").addColumn(Expr(Some("a,c,d"))).tag("select_col").end
println(byzer.toScript)
}
```
It throw exception like this:
```
next on empty iterator
java.util.NoSuchElementException: next on empty iterator
at scala.collection.Iterator$$anon$2.next(Iterator.scala:41)
at scala.collection.Iterator$$anon$2.next(Iterator.scala:39)
at scala.collection.IndexedSeqLike$Elements.next(IndexedSeqLike.scala:63)
at scala.collection.IterableLike.head(IterableLike.scala:109)
at scala.collection.IterableLike.head$(IterableLike.scala:108)
at scala.collection.mutable.ArrayBuffer.scala$collection$IndexedSeqOptimized$$super$head(ArrayBuffer.scala:49)
at scala.collection.IndexedSeqOptimized.head(IndexedSeqOptimized.scala:129)
at scala.collection.IndexedSeqOptimized.head$(IndexedSeqOptimized.scala:129)
at scala.collection.mutable.ArrayBuffer.head(ArrayBuffer.scala:49)
at scala.collection.TraversableLike.last(TraversableLike.scala:519)
at scala.collection.TraversableLike.last$(TraversableLike.scala:518)
at scala.collection.mutable.ArrayBuffer.scala$collection$IndexedSeqOptimized$$super$last(ArrayBuffer.scala:49)
at scala.collection.IndexedSeqOptimized.last(IndexedSeqOptimized.scala:135)
at scala.collection.IndexedSeqOptimized.last$(IndexedSeqOptimized.scala:135)
at scala.collection.mutable.ArrayBuffer.last(ArrayBuffer.scala:49)
at tech.mlsql.byzer_client_sdk.scala_lang.generator.Byzer.lastTableName(byzer.scala:280)
at tech.mlsql.byzer_client_sdk.scala_lang.generator.node.Columns.(Columns.scala:18)
at tech.mlsql.byzer_client_sdk.scala_lang.generator.Byzer.columns(byzer.scala:252)
at tech.mlsql.test.byzer_client_sdk.scala_lang.generator.ByzerScriptTest.$anonfun$new$21(ByzerScriptTest.scala:429)
at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
at org.scalatest.OutcomeOf.outcomeOf(OutcomeOf.scala:85)
at org.scalatest.OutcomeOf.outcomeOf$(OutcomeOf.scala:83)
at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
at org.scalatest.Transformer.apply(Transformer.scala:22)
at org.scalatest.Transformer.apply(Transformer.scala:20)
at org.scalatest.funsuite.AnyFunSuiteLike$$anon$1.apply(AnyFunSuiteLike.scala:226)
at org.scalatest.TestSuite.withFixture(TestSuite.scala:196)
at org.scalatest.TestSuite.withFixture$(TestSuite.scala:195)
at org.scalatest.funsuite.AnyFunSuite.withFixture(AnyFunSuite.scala:1563)
at org.scalatest.funsuite.AnyFunSuiteLike.invokeWithFixture$1(AnyFunSuiteLike.scala:224)
at org.scalatest.funsuite.AnyFunSuiteLike.$anonfun$runTest$1(AnyFunSuiteLike.scala:236)
at org.scalatest.SuperEngine.runTestImpl(Engine.scala:306)
at org.scalatest.funsuite.AnyFunSuiteLike.runTest(AnyFunSuiteLike.scala:236)
at org.scalatest.funsuite.AnyFunSuiteLike.runTest$(AnyFunSuiteLike.scala:218)
at org.scalatest.funsuite.AnyFunSuite.runTest(AnyFunSuite.scala:1563)
at org.scalatest.funsuite.AnyFunSuiteLike.$anonfun$runTests$1(AnyFunSuiteLike.scala:269)
at org.scalatest.SuperEngine.$anonfun$runTestsInBranch$1(Engine.scala:413)
at scala.collection.immutable.List.foreach(List.scala:392)
at org.scalatest.SuperEngine.traverseSubNodes$1(Engine.scala:401)
at org.scalatest.SuperEngine.runTestsInBranch(Engine.scala:396)
at org.scalatest.SuperEngine.runTestsImpl(Engine.scala:475)
at org.scalatest.funsuite.AnyFunSuiteLike.runTests(AnyFunSuiteLike.scala:269)
at org.scalatest.funsuite.AnyFunSuiteLike.runTests$(AnyFunSuiteLike.scala:268)
at org.scalatest.funsuite.AnyFunSuite.runTests(AnyFunSuite.scala:1563)
at org.scalatest.Suite.run(Suite.scala:1114)
at org.scalatest.Suite.run$(Suite.scala:1096)
at org.scalatest.funsuite.AnyFunSuite.org$scalatest$funsuite$AnyFunSuiteLike$$super$run(AnyFunSuite.scala:1563)
at org.scalatest.funsuite.AnyFunSuiteLike.$anonfun$run$1(AnyFunSuiteLike.scala:273)
at org.scalatest.SuperEngine.runImpl(Engine.scala:535)
at org.scalatest.funsuite.AnyFunSuiteLike.run(AnyFunSuiteLike.scala:273)
at org.scalatest.funsuite.AnyFunSuiteLike.run$(AnyFunSuiteLike.scala:272)
at org.scalatest.funsuite.AnyFunSuite.run(AnyFunSuite.scala:1563)
at org.scalatest.tools.SuiteRunner.run(SuiteRunner.scala:47)
at org.scalatest.tools.Runner$.$anonfun$doRunRunRunDaDoRunRun$13(Runner.scala:1321)
at org.scalatest.tools.Runner$.$anonfun$doRunRunRunDaDoRunRun$13$adapted(Runner.scala:1315)
at scala.collection.immutable.List.foreach(List.scala:392)
at org.scalatest.tools.Runner$.doRunRunRunDaDoRunRun(Runner.scala:1315)
at org.scalatest.tools.Runner$.$anonfun$runOptionallyWithPassFailReporter$24(Runner.scala:992)
at org.scalatest.tools.Runner$.$anonfun$runOptionallyWithPassFailReporter$24$adapted(Runner.scala:970)
at org.scalatest.tools.Runner$.withClassLoaderAndDispatchReporter(Runner.scala:1481)
at org.scalatest.tools.Runner$.runOptionallyWithPassFailReporter(Runner.scala:970)
at org.scalatest.tools.Runner$.run(Runner.scala:798)
at org.scalatest.tools.Runner.run(Runner.scala)
at org.jetbrains.plugins.scala.testingSupport.scalaTest.ScalaTestRunner.runScalaTest2or3(ScalaTestRunner.java:38)
at org.jetbrains.plugins.scala.testingSupport.scalaTest.ScalaTestRunner.main(ScalaTestRunner.java:25)

```
It looks like when I add first node in Byzer builder, it can not call [lastTableName] correctly.