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

[aki1770-del]

Closes #4824.

Problem

Scanning a container image that contains an empty .rockspec file causes syft to panic with runtime error: index out of range [0] with length 0.

The bug is in parseRockspecBlock at syft/pkg/cataloger/lua/rockspec_parser.go:66:

if *i >= len(data) && len(out) > 0 {
    return nil, fmt.Errorf("unexpected end of block at %d", *i)
}
c := data[*i]

The guard condition requires len(out) > 0 — for an empty file, out is empty and data is empty, so the guard falls through to c := data[0], which panics.

Fix

Split the guard so empty input returns gracefully:

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]

Preserves the existing "unexpected end of block" error for partial content; returns an empty block cleanly for empty or whitespace-only input.

Tests

Added two cases to rockspec_parser_test.go: empty file and whitespace only — both expect no error and empty result.