[SPARK-58680][CORE] Use isEmpty/nonEmpty for emptiness checks in ExternalAppendOnlyMap

[uros-b]

What changes were proposed in this pull request?

Uses the idiomatic emptiness checks for the three ArrayBuffer length comparisons in ExternalAppendOnlyMap:

if (kcPairs.nonEmpty) {                  // was kcPairs.length > 0
def isEmpty: Boolean = pairs.isEmpty     // was pairs.length == 0
assert(pairs.nonEmpty)                   // was assert(pairs.length > 0)

Why are the changes needed?

The same file already uses this form for its other emptiness checks (spilledMaps.isEmpty, mergeHeap.nonEmpty, mergeHeap.isEmpty), as does the sibling ExternalSorter, so these three were the odd ones out.

To be clear about scope: this is a readability change, not a performance optimization. For ArrayBuffer both forms are O(1) and exactly equivalent -- isEmpty resolves to SeqOps.isEmpty, which is lengthCompare(0) == 0 over the buffer's cached size -- so there is no measurable difference either way.

The currentMap.size > 0 check in forceSpill is deliberately not changed. AppendOnlyMap is an Iterable rather than a Seq and does not override knownSize, so isEmpty there would fall through to iterator(), which asserts the map has not been destructively sorted. Since forceSpill can run under memory pressure after destructiveSortedIterator has been called, that site must stay a size comparison.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Behavior-preserving, so the existing ExternalAppendOnlyMapSuite coverage applies, in particular the spill and hash-collision tests that drive StreamBuffer with more than one pair per hash. The three predicates have identical truth values for every possible buffer state, so no branch can be taken differently.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 4.8)

[uros-b]

Waiting for CI

[dongjoon-hyun]

+1, LGTM