fix(ui): prevent literal "undefined" class name in useDefinition

When no className prop was passed to a component using useDefinition,
the hook would add a literal "undefined" string as a CSS class. This
happened because clsx treats object keys as class names, and
`{ [undefined]: true }` creates an "undefined" key.

Fixed by using conditional expressions instead of object syntax,
which clsx correctly ignores when falsy.

Signed-off-by: Johan Persson <johanopersson@gmail.com>

.changeset/gold-lions-stick.md

@@ -0,0 +1,5 @@
+---
+'@backstage/ui': patch
+---
+
+Fixed `useDefinition` hook adding literal "undefined" class name when no className prop was passed.

packages/ui/src/hooks/useDefinition/useDefinition.tsx

@@ -103,10 +103,8 @@ export function useDefinition<
     classes[name] = clsx(
       cssKey as string,
       definition.styles[cssKey as keyof typeof definition.styles],
-      {
-        [utilityClasses]: utilityTarget === name,
-        [ownPropsResolved.className]: classNameTarget === name,
-      },
+      utilityTarget === name && utilityClasses,
+      classNameTarget === name && ownPropsResolved.className,
     );
   }