Get the node title conditionally in a view rewrite field
Is title a field you made or do you mean the node title? Node title can be printed with either {{ label }} or {{ node.getTitle() }}.

Fields that you created on the node, they are rendered out as:

{{ content.field_title }}

Which can be checked as:

{% if node.field_title_external is not empty  %}
  {{ content.field_title_external }}
{% else %}
  {{ node.getTitle() }}
{% endif %}


You can also use {{ label }} instead of {{ node.getTitle() }}.