internal/zstd: allow empty compressed blocks

For golang#62513

src/internal/zstd/block.go

@@ -50,10 +50,9 @@ func (r *Reader) compressedBlock(blockSize int) error {
 		if off < len(data) {
 			return r.makeError(off, "extraneous data after no sequences")
 		}
-		if len(litbuf) == 0 {
-			return r.makeError(off, "no sequences and no literals")
-		}
+
 		r.buffer = append(r.buffer, litbuf...)
+
 		return nil
 	}
 
@@ -374,9 +373,7 @@ func (r *Reader) execSeqs(data block, off int, litbuf []byte, seqCount int) erro
 		}
 	}
 
-	if len(litbuf) > 0 {
-		r.buffer = append(r.buffer, litbuf...)
-	}
+	r.buffer = append(r.buffer, litbuf...)
 
 	if rbr.cnt != 0 {
 		return r.makeError(off, "extraneous data after sequences")

src/internal/zstd/zstd_test.go

@@ -90,6 +90,11 @@ var tests = []struct {
 		"0\x00\x00\x00\x00\x000\x00\x00\x00\x00\x001\x00\x00\x00\x00\x000000",
 		"(\xb5/\xfd\x04X\x8d\x00\x00P0\x000\x001\x000000\x03T\x02\x00\x01\x01m\xf9\xb7G",
 	},
+	{
+		"empty block",
+		"",
+		"\x28\xb5\x2f\xfd\x00\x00\x15\x00\x00\x00\x00",
+	},
 }
 
 func TestSamples(t *testing.T) {