[To dev/1.3]Fix fast last empty cache handling

[Caideyipi]

Description

Motivation

In 1.3, fast last query may use DeviceLastCache.EMPTY_TIME_VALUE_PAIR to represent a cache hit whose last value is
empty.

The prefix-path fast last query path already skips this empty cache entry, but
executeFastLastDataQueryForOneDeviceV2 only checked whether timeValuePair.getValue() was null. As a result,
DeviceLastCache.EMPTY_TIME_VALUE_PAIR could be handled as a normal cached value.

Modification

This PR updates the one-device fast last query cache-hit path to treat DeviceLastCache.EMPTY_TIME_VALUE_PAIR as an
empty last-cache entry.

Tests

mvn -pl iotdb-core/datanode -am -DskipTests compile

Key changed/added classes in this PR

• ClientRPCServiceImpl

---

This PR has:

• been self-reviewed.
  • concurrent read
  • concurrent write
  • concurrent read and write
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods.
• added or updated version, license, or notice information
• added comments explaining the "why" and the intent of the code wherever would not be obvious
  for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold
  for code coverage.
• added integration tests.
• been tested in a test IoTDB cluster.

---

Key changed/added classes (or packages if there are too many classes) in this PR

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.10%. Comparing base (8657782) to head (e758265).

Files with missing lines	Patch %	Lines
.../db/protocol/thrift/impl/ClientRPCServiceImpl.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             dev/1.3   #17743      +/-   ##
=============================================
- Coverage      42.10%   42.10%   -0.01%     
  Complexity       241      241              
=============================================
  Files           3613     3613              
  Lines         238754   238755       +1     
  Branches       29004    29005       +1     
=============================================
- Hits          100533   100522      -11     
- Misses        138221   138233      +12     

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