Fix last_by returning null row after deleting all table data (#16985)

[PDGGK]

Description

Fixes #16985

In table model, after deleting all data of a device, last_by queries still returned one row with null values instead of returning no rows:

-- After deleting all data:
select last_by(s0, time) from table where deviceId = 'd0'
-- Expected: 0 rows
-- Actual: 1 row with null

Root Cause

Three related issues:

1. LastQueryAggTableScanOperator could still emit a result row for non-grouped last_by scans even when every LastByAccumulator had no initialized result, causing evaluateFinal() to produce a null row.

2. FileLoaderUtils read aligned time-column modifications using the concrete measurement ID instead of AlignedFullPath.VECTOR_PLACEHOLDER, inconsistent with aligned deletion semantics used in other paths.

3. ModificationUtils did not set modified = true when an aligned value chunk was fully deleted, allowing statistics-based shortcuts to use stale metadata.

Changes

File	Change
LastQueryAggTableScanOperator.java	Skip emitting result row when all aggregators are LastByAccumulator and none has initialized result. Applied to both normal aggregation and cache-hit paths.
FileLoaderUtils.java	Use AlignedFullPath.VECTOR_PLACEHOLDER for time-column modification lookups
ModificationUtils.java	Set modified = true when value chunk is fully removed
IoTDBDeletionTableIT.java	Add regression tests for 2-arg and 3-arg last_by after delete

Semantic Note

This changes non-grouped last_by on empty results from returning one null row to returning zero rows, consistent with grouped-query behavior (which already returns zero rows for empty devices). Other aggregations like count/max/last are not affected by this change. Please confirm this is the intended semantics.

Verification

Integration tests pass locally:

mvn -P with-integration-tests,TableSimpleIT -pl integration-test -am \
  -DskipUTs -Dspotless.skip=true -Dcheckstyle.skip=true -Drat.skip=true \
  -Dit.test=IoTDBDeletionTableIT#testLastByWithTimeShouldReturnNoRowsAfterDeletingAllData+testThreeArgLastByShouldReturnNoRowsAfterDeletingAllData \
  verify

Result: 2 tests run, 0 failures, 0 errors.