Empty XMP content when passing kXMPFiles_OpenOnlyXMP on video file

I'm using [the Rust crate](https://github.com/adobe/xmp-toolkit-rs) to reproduce it, so the following reproduce code is in Rust. I'm still unsure whether this behavior is a bug or expected (It’s also possible that the video file I'm working with is non-standard. I can provide a test file if you'd like).

### Expected Behaviour
xmp content should be read by passing kXMPFiles_OpenOnlyXMP

### Actual Behaviour
with kXMPFiles_OpenOnlyXMP, program returns xmp with empty content (when reading mp4 video files)

### Reproduce Scenario

The input media is a mp4 video file, with only_xmp:
```rust
let mut media_xmp = XmpFile::new()?;
if media_xmp
.open_file(media.clone(), OpenFileOptions::default().only_xmp())
.is_ok()
{
if let Some(xmp) = media_xmp.xmp() {
xmp_string = xmp.to_string_with_options(
ToStringOptions::default().set_newline("\n".to_string()),
)?;
println!("XMP: {}", xmp_string)
}
```

Output

```




























```

without only_xmp:
```rust
let mut media_xmp = XmpFile::new()?;
if media_xmp
.open_file(media.clone(), OpenFileOptions::default().only_xmp())
.is_ok()
{
if let Some(xmp) = media_xmp.xmp() {
xmp_string = xmp.to_string_with_options(
ToStringOptions::default().set_newline("\n".to_string()),
)?;
println!("XMP: {}", xmp_string)
}
```

Output

```
XMP:



2024-06-02T13:11:36Z
2024-06-02T13:11:36Z

20187
1/1000


























```

Start with the Rust reproduction using XmpFile::open_file and OpenFileOptions::only_xmp() against the reported MP4 input, then compare its output with the same call without only_xmp. Confirm the behavior with a representative test video or the author's test file; done means determining whether XMP content should be returned and documenting or reproducing the result in the SDK.

Written by the indexing model from the issue text.