Improve JNDI detection logic

Update JndiLocatorDelegate.isDefaultJndiEnvironmentAvailable() to
call `getEnvironment()` on the `InitialContext` in order to actually
trigger a NamingException if JNDI is not available.

Issue: SPR-12223

spring-context/src/main/java/org/springframework/jndi/JndiLocatorDelegate.java

@@ -58,7 +58,7 @@ public class JndiLocatorDelegate extends JndiLocatorSupport {
 	 */
 	public static boolean isDefaultJndiEnvironmentAvailable() {
 		try {
-			new InitialContext();
+			new InitialContext().getEnvironment();
 			return true;
 		}
 		catch (Throwable ex) {