This is a simple helper template, to be used in other templates. It displays a link to a category if that category exists; otherwise it just displays greyed-out plaintext.
Optional display text may be added as a second parameter.
{{LinkCatIfExists|CategoryName|OptionalDisplayText}}
{{LinkCatIfExists|CategoryName}}
{{LinkCatIfExists|A non-existent category}}
→ A non-existent category{{LinkCatIfExists|United States}}
→ United States{{LinkCatIfExists|CategoryName|DisplayText}}
{{LinkCatIfExists|A non-existent category|Foo}}
→ Foo{{LinkCatIfExists|United States|USA}}
→ USA