[SPARK-8621] [SQL] support empty string as column name

improve the empty check in `parseAttributeName` so that we can allow empty string as column name.
Close https://github.com/apache/spark/pull/7117

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #7149 from cloud-fan/8621 and squashes the following commits:

efa9e3e [Wenchen Fan] support empty string

1 files changed, 7 insertions, 0 deletions

diff --git a/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala
index 50d324c068..afb1cf5f8d 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala
@@ -730,4 +730,11 @@ class DataFrameSuite extends QueryTest {
     val res11 = ctx.range(-1).select("id")
     assert(res11.count == 0)
   }
+
+  test("SPARK-8621: support empty string column name") {
+    val df = Seq(Tuple1(1)).toDF("").as("t")
+    // We should allow empty string as column name
+    df.col("")
+    df.col("t.``")
+  }
 }