fix: check for changes array during author step (#44)

There is already a try/catch block so the effective result is the same,
but at least it's an if statement now instead of an ignored exception.

lib/normalize.js

@@ -154,7 +154,7 @@ const normalize = async (pkg, { strict, steps, root, changes, allowLegacyCase })
         .map(line => line.replace(/^\s*#.*$/, '').trim())
         .filter(line => line)
       data.contributors = authors
-      changes.push('"contributors" was auto-populated with the contents of the "AUTHORS" file')
+      changes?.push('"contributors" was auto-populated with the contents of the "AUTHORS" file')
     } catch {
       // do nothing
     }