[MINOR][SQL] Pattern match using Singleton to check if object is a scala object

[techaddict]

What changes were proposed in this pull request?

To Check if obj is an Scala Object, Instead of doing

case obj if obj.getClass.getName.endsWith("$")

we do

case obj: Singleton

How was this patch tested?

Existing tests

[techaddict]

cc: @cloud-fan

[SparkQA]

Test build #58501 has finished for PR 13080 at commit b9e6eab.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.