I had a similar issue where the user's name was either their name or their email address

Instead of having a separate field, why not utilize the requester's name?

I created a liquid-markup dynamic content field that accomplishes something similar

I named the Dynamic content "name" so referencing it is {{dc.name}} on all macros and content

{% if ticket.requester.email == ticket.requester.name %} there{% else %} {{ticket.requester.first_name}}{% endif %}

Example: Hi {{dc.name}},

produces, Hi there, or Hi McCabe,

Similarly you could remove the "there" portion and leave it blank