fix(tsdb): Fix tsm1 block merge. (#14254)

fix(tsdb): Fix tsm1 block merge.

tsdb/tsm1/reader_block_iterator.go

@@ -10,6 +10,9 @@ type BlockIterator struct {
 
 // PeekNext returns the next key to be iterated or an empty string.
 func (b *BlockIterator) PeekNext() []byte {
+	if len(b.entries) > 1 {
+		return b.iter.Key()
+	}
 	return b.iter.Peek()
 }