[SPARK-28331][SQL] Catalogs.load() should be able to load built-in catalogs

## What changes were proposed in this pull request?

In `Catalogs.load`, the `pluginClassName` in the following code
```
String pluginClassName = conf.getConfString("spark.sql.catalog." + name, null);
```
is always null for built-in catalogs, e.g there is a SQLConf entry `spark.sql.catalog.session`.

This is because of #18852: SQLConf.conf.getConfString(key, null) always returns null.

## How was this patch tested?

Apply code changes of #24768 and tried loading session catalog.

Closes #25094 from gengliangwang/fixCatalogLoad.

Authored-by: Gengliang Wang <[email protected]>
Signed-off-by: Burak Yavuz <[email protected]>

Author: gengliangwang

sql/catalyst/src/main/java/org/apache/spark/sql/catalog/v2/Catalogs.java

@@ -26,6 +26,7 @@
 import java.lang.reflect.InvocationTargetException;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.NoSuchElementException;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -50,8 +51,10 @@ private Catalogs() {
    */
   public static CatalogPlugin load(String name, SQLConf conf)
       throws CatalogNotFoundException, SparkException {
-    String pluginClassName = conf.getConfString("spark.sql.catalog." + name, null);
-    if (pluginClassName == null) {
+    String pluginClassName;
+    try {
+      pluginClassName = conf.getConfString("spark.sql.catalog." + name);
+    } catch (NoSuchElementException e){
       throw new CatalogNotFoundException(String.format(
           "Catalog '%s' plugin class not found: spark.sql.catalog.%s is not defined", name, name));
     }