fix: guard null partition location in weak read prep

[maochongxin]

Background

In origin/master, ObPartitionEntry.prepareForWeakRead() iterates over partitionInfos and directly calls entry.getValue().getPartitionLocation().prepareForWeakRead(ldcLocation).

However, getPartitionInfo() creates and publishes an ObPartitionLocationInfo via ConcurrentHashMap.computeIfAbsent(). At that moment, partitionLocation may still be null.

During concurrent tablet location refreshes, one thread may iterate over an entry that was just published by another thread but has not been populated by updateLocation() yet, causing an NPE.

Changes

• Add null checks for ObPartitionLocationInfo and ObPartitionLocation in prepareForWeakRead().
• Skip partition locations that have not been initialized yet.
• Add a regression test covering the case where an empty ObPartitionLocationInfo has been published but its partitionLocation is still null.

[CLAassistant]

All committers have signed the CLA.

[WeiXinChan]

LGTM