compaction: tolerate empty keys

The compaction iterator will sometimes convert empty keys with nil.
Apparently, we want to allow empty keys (but not nil keys).

We make a small change to the compaction iterator to avoid this corner
case and we update `base.MinUserKey` to treat empty non-nil keys as
regular keys.

compaction.go

@@ -2861,6 +2861,9 @@ func (d *DB) runCompaction(
 		if key != nil && firstKey == nil {
 			firstKey = key.UserKey
 		}
+		if invariants.Enabled && firstKey == nil {
+			panic("nil first key")
+		}
 		splitter := compact.NewOutputSplitter(c.cmp, firstKey, splitLimitFunc(firstKey), c.maxOutputFileSize, c.grandparents.Iter(), iter.Frontiers())
 		if err := newOutput(); err != nil {
 			return nil, pendingOutputs, stats, err

internal/base/comparer.go

@@ -286,10 +286,10 @@ func SharedPrefixLen(a, b []byte) int {
 	return i
 }
 
-// MinUserKey returns the smaller of two user keys. If one of the keys is empty,
+// MinUserKey returns the smaller of two user keys. If one of the keys is nil,
 // the other one is returned.
 func MinUserKey(cmp Compare, a, b []byte) []byte {
-	if len(a) > 0 && (len(b) == 0 || cmp(a, b) < 0) {
+	if a != nil && (b == nil || cmp(a, b) < 0) {
 		return a
 	}
 	return b

internal/compact/iterator.go

@@ -307,6 +307,9 @@ func NewIter(
 	i := &Iter{
 		cmp: cfg.Comparer.Compare,
 		cfg: cfg,
+		// We don't want a nil keyBuf because if the first key we encounter is
+		// empty, it would become nil.
+		keyBuf: make([]byte, 8),
 	}
 
 	iter := pointIter