colblk: fix UintBuilder's construction of constant column with default

Previously if a UintBuilder was constructed with InitWithDefault, and the
column contained no non-zero elements, the UintBuilder would never construct an
array to back b.array.elems. The call to unsafe.Slice would panic when it
observed a non-zero element count and a nil pointer.

sstable/colblk/testdata/uints

@@ -4,6 +4,23 @@
 # be finished separately so the test can continue with testing higher-width
 # integers.
 
+# Test a default-zero builder that only contains zeros.
+
+init widths=(32) default-zero
+----
+b32
+
+size rows=(100)
+----
+b32:
+  32: *colblk.UintBuilder[uint32].Size(100, 0) = 5
+
+finish widths=(32) rows=100
+----
+b32: *colblk.UintBuilder[uint32]:
+0-1: x 01       # delta encoding: const
+1-5: x 00000000 # 32-bit constant: 0
+
 # Initialize all four writers.
 
 init widths=(8, 16, 32, 64)

sstable/colblk/uints.go

@@ -297,7 +297,18 @@ func (b *UintBuilder[T]) Finish(col, rows int, offset uint32, buf []byte) uint32
 		minimum, maximum = computeMinMax(b.array.elems.Slice(rows))
 		w = deltaWidth(uint64(maximum - minimum))
 	}
-	return uintColumnFinish[T](minimum, b.array.elems.Slice(rows), w, offset, buf)
+
+	// NB: In some circumstances, it's possible for b.array.elems.ptr to be nil.
+	// Specifically, if the builder is initialized using InitWithDefault and no
+	// non-default values exist, no array will have been allocated (we lazily
+	// allocate b.array.elems.ptr). It's illegal to try to construct an unsafe
+	// slice from a nil ptr with non-zero rows. Only attempt to construct the
+	// values slice if there's actually a non-nil ptr.
+	var valuesSlice []T
+	if b.array.elems.ptr != nil {
+		valuesSlice = b.array.elems.Slice(rows)
+	}
+	return uintColumnFinish[T](minimum, valuesSlice, w, offset, buf)
 }
 
 // uintColumnFinish finishes the column of unsigned integers of type T, encoding