fix(api-docs): set url explicitly to an empty string

The url argument is undefined when it is not explicitly set to an empty string. An undefined url is
causing the swagger UI to fail on match checks.

fixes #8895

Signed-off-by: Dominik Schwank <dominik.schwank@sda.se>

.changeset/sour-spies-hug.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-api-docs': patch
+---
+
+Using an explicitly empty string for the url argument ensures that the swagger UI does not run into undefined errors.

plugins/api-docs/src/components/OpenApiDefinitionWidget/OpenApiDefinition.tsx

@@ -148,7 +148,7 @@ export const OpenApiDefinition = ({ definition }: OpenApiDefinitionProps) => {
 
   return (
     <div className={classes.root}>
-      <SwaggerUI spec={def} deepLinking />
+      <SwaggerUI spec={def} url="" deepLinking />
     </div>
   );
 };