explicitly check for DataElement before reference value

[wetzelj]

Description

Related issues: #256

Updated filter.py empty() function to explicitly check for the tag content being an instance of a DataElement instead of assuming it's a DataElement if it isn't a string.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• My code follows the style guidelines of this project

Open questions

None

[wetzelj]

Thanks @vsoch!

[vsoch]

Thank you @wetzelj !!