[SPARK-25058][SQL] Use Block.isEmpty/nonEmpty to check whether the code is empty or not.

[ueshin]

What changes were proposed in this pull request?

We should use Block.isEmpty/nonEmpty instead of comparing with empty string to check whether the code is empty or not.

[error] [warn] /.../sql/core/src/main/scala/org/apache/spark/sql/execution/WholeStageCodegenExec.scala:278: org.apache.spark.sql.catalyst.expressions.codegen.Block and String are unrelated: they will most likely always compare unequal
[error] [warn]       if (ev.code != "" && required.contains(attributes(i))) {
[error] [warn] 
[error] [warn] /.../sql/core/src/main/scala/org/apache/spark/sql/execution/joins/BroadcastHashJoinExec.scala:323: org.apache.spark.sql.catalyst.expressions.codegen.Block and String are unrelated: they will most likely never compare equal
[error] [warn]          |  ${buildVars.filter(_.code == "").map(v => s"${v.isNull} = true;").mkString("\n")}
[error] [warn] 

How was this patch tested?

Existing tests.

[ueshin]

cc @viirya @cloud-fan @gatorsmile

[kiszk]

LGTM

[viirya]

LGTM

[SparkQA]

Test build #94437 has finished for PR 22041 at commit fa7e1b1.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[ueshin]

Thanks! merging to master.