[SPARK-21112][SQL] ALTER TABLE SET TBLPROPERTIES should not overwrite COMMENT

### What changes were proposed in this pull request?
`ALTER TABLE SET TBLPROPERTIES` should not overwrite `COMMENT` even if the input property does not have the property of `COMMENT`. This PR is to fix the issue.

### How was this patch tested?
Covered by the existing tests.

Author: Xiao Li <gatorsmile@gmail.com>

Closes apache#18318 from gatorsmile/fixTableComment.

Author: gatorsmile

sql/core/src/main/scala/org/apache/spark/sql/execution/command/ddl.scala

@@ -235,7 +235,7 @@ case class AlterTableSetPropertiesCommand(
     // direct property.
     val newTable = table.copy(
       properties = table.properties ++ properties,
-      comment = properties.get("comment"))
+      comment = properties.get("comment").orElse(table.comment))
     catalog.alterTable(newTable)
     Seq.empty[Row]
   }

sql/core/src/test/resources/sql-tests/results/describe.sql.out

@@ -127,6 +127,7 @@ Provider            	parquet
 Num Buckets         	2                   	                    
 Bucket Columns      	[`a`]               	                    
 Sort Columns        	[`b`]               	                    
+Comment             	table_comment       	                    
 Table Properties    	[e=3]               	                    
 Location [not included in comparison]sql/core/spark-warehouse/t	                    
 Storage Properties  	[a=1, b=2]          	                    
@@ -157,6 +158,7 @@ Provider            	parquet
 Num Buckets         	2                   	                    
 Bucket Columns      	[`a`]               	                    
 Sort Columns        	[`b`]               	                    
+Comment             	table_comment       	                    
 Table Properties    	[e=3]               	                    
 Location [not included in comparison]sql/core/spark-warehouse/t	                    
 Storage Properties  	[a=1, b=2]