fix: change rule to use IsNullOrEmpty

[will-larkin-nhs]

Description

Changed transform rule 3 date of death not null with RFR != DEA to use string.IsNullOrEmpty()

Context

The transformation rule was triggering for every participant because the rule used != null instead of !string.IsNullOrEmpty
DTOSS-7811

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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