Ignore the seed hash of an empty B in theta/tuple a-not-b

[jaideeppyne]

While investigating #460 (Java/C++ theta byte differences) I found a related bug that is not about bytes.

An empty sketch retains no hashes, so its seed hash carries no information. Three of the four C++ paths that validate a seed hash already know this:

• compact_theta_sketch::deserialize_v3 / deserialize_v4: if (!is_empty) checker<true>::check_seed_hash(...)
• theta_union_base::update: if (sketch.is_empty()) return; before the check
• theta_intersection_base::update: if (!sketch.is_empty() && sketch.get_seed_hash() != ...) throw

and all three existing seed mismatch tests record the intent in a comment: sketch.update(1); // non-empty should not be ignored.

theta_set_difference_base::compute is the exception. Its early return only fires when a.get_num_retained() > 0, so when A is non-empty with zero retained entries the check is reached and an empty B with a different seed hash throws.

Reproducible in C++ alone, no Java involved:

// B: empty, built with a different seed
auto b = update_theta_sketch::builder().set_seed(12345).build().compact();
// A: non-empty, zero retained (every hash exceeded theta)
auto a = update_theta_sketch::builder().set_p(1e-6f).build();
a.update(1); a.update(2); a.update(3);

compact_theta_sketch::deserialize(...);              // ACCEPTED
theta_union::builder().build().update(b);            // ACCEPTED
theta_intersection().update(b);                      // ACCEPTED
theta_a_not_b().compute(a.compact(), b);             // THREW -> B seed hash mismatch

It also breaks interop with datasketches-java, which serializes every empty compact sketch as the constant {1, 3, 3, 0, 0, 0x1E, 0, 0}. EmptyCompactSketch documents that seed hash of 0 as ignored, so every empty sketch arriving from Java trips this path. In a 20 case Java/C++ differential I ran, all 6 cases that produce an empty sketch failed here and now pass.

The fix guards B's check with !b.is_empty(), matching what theta_intersection_base::update already does. A's check is left alone because A is guaranteed non-empty by the early return above it.

Verification: the added test fails on master with B seed hash mismatch and passes with the fix. Full suite green afterwards, 17/17 ctest suites, 20,245,913 assertions in the theta suite. This code is shared with the tuple sketches, and tuple_test passes too.

I used Claude Code to help run the differential harness and prepare this change. I verified the behavior, the fix and the test results myself against actual build and test output.