[SPARK-15234][SQL] Fix spark.catalog.listDatabases.show()

## What changes were proposed in this pull request?

Before:
```
scala> spark.catalog.listDatabases.show()
+--------------------+-----------+-----------+
|                name|description|locationUri|
+--------------------+-----------+-----------+
|Database[name='de...|
|Database[name='my...|
|Database[name='so...|
+--------------------+-----------+-----------+
```

After:
```
+-------+--------------------+--------------------+
|   name|         description|         locationUri|
+-------+--------------------+--------------------+
|default|Default Hive data...|file:/user/hive/w...|
|  my_db|  This is a database|file:/Users/andre...|
|some_db|                    |file:/private/var...|
+-------+--------------------+--------------------+
```

## How was this patch tested?

New test in `CatalogSuite`

Author: Andrew Or <andrew@databricks.com>

Closes #13015 from andrewor14/catalog-show.