Jj/1798 languages warning (#1805)
### Summary
Closes #1798
Fixes language detection of elements with empty strings: This resolves a
warning message that was raised by `langdetect` if the language was
attempted to be detected on an empty string. Language detection is now
skipped for empty strings.
### Testing
on the main branch this will log the warning "No features in text", but
it will not log anything on this branch.
```
from unstructured.documents.elements import NarrativeText, PageBreak
from unstructured.partition.lang import apply_lang_metadata
elements = [NarrativeText("Sample text."), PageBreak("")]
elements = list(
apply_lang_metadata(
elements=elements,
languages=["auto"],
detect_language_per_element=True,
),
)
```
### Other
Also changes imports in test_lang.py so imports are explicit
---------
Co-authored-by: cragwolfe <crag@unstructured.io>
