fix(lua-rockspec): handle empty and whitespace-only rockspec files gracefully (#4827)

Empty or whitespace-only .rockspec files cause parseRockspecBlock to
panic with "index out of range" because the existing end-of-data guard
requires len(out) > 0 before returning the "unexpected end of block"
error, letting the bare data[*i] access on the next line crash.

Split the guard so that:
  - partial content at end of data still returns the existing error
  - empty data (or whitespace-only) returns an empty block cleanly

Closes #4824.

Signed-off-by: Akihiko Komada <aki1770@gmail.com>

Author: aki1770-del

syft/pkg/cataloger/lua/rockspec_parser.go

@@ -63,8 +63,11 @@ func parseRockspecBlock(data []byte, i *int, locals map[string]string) ([]rocksp
 
 	parsing.SkipWhitespace(data, i)
 
-	if *i >= len(data) && len(out) > 0 {
-		return nil, fmt.Errorf("unexpected end of block at %d", *i)
+	if *i >= len(data) {
+		if len(out) > 0 {
+			return nil, fmt.Errorf("unexpected end of block at %d", *i)
+		}
+		return out, nil
 	}
 
 	c := data[*i]

syft/pkg/cataloger/lua/rockspec_parser_test.go

@@ -14,6 +14,17 @@ func Test_parseRockspecData(t *testing.T) {
 		content string
 		wantErr require.ErrorAssertionFunc
 	}{
+		{
+			name:    "empty file",
+			content: ``,
+		},
+		{
+			name: "whitespace only",
+			content: `
+
+
+`,
+		},
 		{
 			name: "basic valid content",
 			content: `