Describe the bug
This error occurs when xml opening tags are on the same level as the xml context itself

'lxml.etree._ProcessingInstruction' object has no attribute 'is_phrasing'

This breaks

    ```xml
    <?xml version="1.0" encoding="UTF-8"?>

This succeeds

```xml
   <?xml version="1.0" encoding="UTF-8"?>

To Reproduce
Use unstructured to process a markdown file that contains an xml block with the opening tags on the same level as the start of the content

Expected behavior
This should not be a breaking format, it should continue in the same way as is the xml opening tag was tabbed back

Screenshots
If applicable, add screenshots to help explain your problem.

Environment Info
Please run python scripts/collect_env.py and paste the output here.
I am running unstructured version 0.18.24