Template:If then show works great, thanks!
Since the template data does not have any functional feature to actually set it, I wonder what's the convention to set one.
I was using {{{param|defaultvalue}}} syntax, but this doesn't work well if the param is marked as "Suggested" because then the visual editor defines it as an empty string and thus shadows the default value.
{{#if:{{{param|}}}|{{{param}}}|defaultvalue}} does work, but is this really a convention? What do others do?
On Wikipedia there's Template:If then show, which slightly simplifies the syntax for doing exactly the same thing as your {{#if:...}} version, e.g.:
{{If then show|{{{param|}}}|defaultvalue}}
Search this Help page for "default" and "autovalue". One of those may be what you want.
@Cyberkagami: Yes, {{#if:{{{param|}}}|{{{param}}}|defaultvalue}} is a convention. It not only helps with VisualEditor and TemplateData, but also makes possible to create copy-pastable “skeletons” in old-fashioned template documentation, like this one:
{{Infobox foo
|param =
|bar =
|baz =
}}
which would also be broken by the {{{param|defaultvalue}}} syntax if unused parameters are not entirely removed.