Fix fast last query handling for empty last-cache entries

[Caideyipi]

Description

Motivation

Fast last query cache may contain TableDeviceLastCache.PLACEHOLDER_EMPTY_COLUMN for measurements that have no last
value. Some cache-hit response paths only checked whether the cached value was null, so the placeholder entry could
be handled as a normal cached value.

Modification

This PR treats PLACEHOLDER_EMPTY_COLUMN as an empty last-cache entry in fast last query response building.

Changes include:

• Skip placeholder empty-column entries when building REST fast last query responses.
• Treat placeholder empty-column entries the same as null-value cache entries in
  executeFastLastDataQueryForOneDeviceV2.

Key changed/added classes in this PR

• ClientRPCServiceImpl
• RestApiServiceImpl

---

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 40.59%. Comparing base (c1bbbd1) to head (3f35dc9).

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

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17742      +/-   ##
============================================
+ Coverage     40.47%   40.59%   +0.12%     
  Complexity     2574     2574              
============================================
  Files          5179     5179              
  Lines        349942   349943       +1     
  Branches      44737    44738       +1     
============================================
+ Hits         141631   142072     +441     
+ Misses       208311   207871     -440     

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