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