Fix theta preamble count for legacy single-item images

[jaideeppyne]

Closes #773.

Historical C++ theta single-item images may omit the optional single-item flag. Java accepts these images, but PreambleUtil.preambleToString() previously derived the implied count from that flag and displayed CurrentCount: 0.

This change derives the one-preamble-long count from the authoritative empty flag while continuing to display the serialized single-item flag unchanged.

The regression test clears only the flag on a valid single-item image and verifies:

• SINGLE_ITEM: false accurately reflects the historical bytes
• CurrentCount: 1 accurately reflects the retained item

Validation:

• mvn -t /private/tmp/jdk25-toolchains.xml -Dtest=org.apache.datasketches.theta.PreambleUtilTest test (8 tests)
• mvn -t /private/tmp/jdk25-toolchains.xml test (2,287 tests)
• git diff --check

[leerho]

This correctly fixes the preambleToString() single-item/empty count logic from issue #460—the one-line change is sound, CI passes, and the test properly validates the bug fix,