fix/bad formed formula (#1481)
@ron-unstructured reported that loading files with:
```
from unstructured.partition.pdf import partition_pdf
elements_yolox = partition_pdf(filename="1706.03762.pdf", strategy='hi_res', model_name="yolox")
print(elements_yolox)
```
Throws an error. After debugging the execution I found that the issue is
that an object of class Formula is being created, however, this class
doesn't contain an __init__ method. This PR solves the issue of adding a
constructor method with an empty string for the element.
The file can be found at:
https://drive.google.com/drive/folders/1hDumyps0hA4_d-GZxs3Hij15Cpa5fjWY?usp=sharing
After this PR is merged this file is correctly processed
