What is the proper way to check if a variable is undef in a puppet template?

In the manifest the variable is defined as follows

$myvar = undef

How is this checked in the template?

Is saw the following two variants

<% if @myvar -%>
<% end -%>

and

<% if not @myvar.nil? and @myvar -%>
<% end -%>

They both seem to work in my case, but I wonder if the first approach fails in on certain cases?

erb puppet
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