Fix ArgumentNullException in HashBlockLevelContent when Unid attribute is missing

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This PR fixes an ArgumentNullException that occurs in HashBlockLevelContent when a block-level element (<w:p>, <w:tbl>, <w:tr>) does not have a Unid attribute.

• Added a null check for the Unid attribute when building the dictionary, so only elements with a non-null Unid are included.
• This prevents the code from attempting to use a null key in the dictionary, which caused the exception.

Why:
Without this fix, comparing certain Word documents could result in a crash due to missing Unid attributes.

How to test:

• Run a comparison on Word files that previously caused the exception.
• The process should now complete without errors.