fix Next.js SDK type error: default undefined rewrites to empty arrays

Next.js 14 types require definite Rewrite[] arrays in the rewrites
return type. The beforeFiles/afterFiles/fallback fields were
potentially undefined. Default to [] to satisfy the type checker.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: BrewingCoder

sdk/highlight-next/src/util/with-highlight-config.ts

@@ -172,11 +172,11 @@ const getHighlightConfig = async (
 				return {
 					beforeFiles: defaultOpts.uploadSourceMaps
 						? (re.beforeFiles ?? []).concat(sourcemapRewrite)
-						: re.beforeFiles,
+						: re.beforeFiles ?? [],
 					afterFiles: defaultOpts.configureHighlightProxy
 						? (re.afterFiles ?? []).concat(...highlightRewrites)
-						: re.afterFiles,
-					fallback: re.fallback,
+						: re.afterFiles ?? [],
+					fallback: re.fallback ?? [],
 				}
 			}
 		}