ZOOKEEPER-5051: Avoid NPE closing 4lw connection during startup

[PAC-MAJ]

Problem

When a 4-letter command is sent while the ZooKeeper server is starting,
the connection close path can call ZooKeeperServer.removeCnxn() before
startdata() has initialized zkDb.

This can trigger:

java.lang.NullPointerException: Cannot invoke
"org.apache.zookeeper.server.ZKDatabase.removeCnxn(...)"
because "this.zkDb" is null

But this also hangs any client waiting for connection close.

Fix

Guard ZooKeeperServer.removeCnxn() so it only delegates to zkDb
when zkDb has already been initialized.

Test

Added ZooKeeperServerTest#testRemoveCnxnBeforeStartData.
The test creates a ZooKeeperServer but intentionally does not call
startdata(), then verifies that removeCnxn() does not throw.

[PAC-MAJ]

Hi ZooKeeper maintainers,

Gentle ping on this PR for ZOOKEEPER-5051.

This fixes a startup race where a 4lw command connection can be closed before ZooKeeperServer.startdata() initializes zkDb, causing ZooKeeperServer.removeCnxn() to throw an NPE and potentially leave the client connection hanging.

The change is intentionally small: guard removeCnxn() when zkDb is not initialized yet. I also added a deterministic regression test for the pre-startdata case.

The previous precommit failure looked like Jenkins infrastructure/agent failure during result collection rather than a test failure. Could someone please rerun CI or advise if anything else is needed from my side?

Thanks!