RFR: JDK-8276939: Fix AbstractLdapNamingEnumeration next to throw NoSuchElementException instead of NullPointerException

[andrewluotechnologies]

`AbstractLdapNamingEnumeration` `next` throws `NullPointerException` instead of `NoSuchElementException`, however the javadoc for `NamingEnumeration` says that next should throw `NoSuchElementException` when no elements are available (https://docs.oracle.com/en/java/javase/17/docs/api/java.naming/javax/naming/NamingEnumeration.html#next())

The bug is basically that `next()` calls `hasMore()` -> `hasMoreImpl()` which calls `cleanup()` when there are no more elements.  `cleanup()` sets `homeCtx = null;` which causes NullPointerException to be thrown on this line in `getNextBatch()`: `res = homeCtx.getSearchReply(enumClnt, res);`

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Commit messages:
 - JDK-8276939: Fix AbstractLdapNamingEnumeration next to throw NoSuchElementException instead of NullPointerException

Changes: https://git.openjdk.java.net/jdk/pull/6095/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6095&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276939
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6095.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6095/head:pull/6095

PR: https://git.openjdk.java.net/jdk/pull/6095