Add category title to changelog only if present; avoid "undefined" if not specified.

src/transform.ts

@@ -173,14 +173,18 @@ export function buildChangelog(diffInfo: DiffInfo, prs: PullRequestInfo[], optio
   let changelog = ''
   for (const [category, pullRequests] of categorized) {
     if (pullRequests.length > 0) {
+      if (category.title) {
         changelog = `${changelog + category.title}\n\n`
+      }
 
       for (const pr of pullRequests) {
         changelog = `${changelog + pr}\n`
       }
       changelog = `${changelog}\n` // add space between sections
     } else if (category.empty_content !== undefined) {
+      if (category.title) {
         changelog = `${changelog + category.title}\n\n`
+      }
       changelog = `${changelog + category.empty_content}\n\n`
     }
   }