fix(cfnspec): incorrectly handling array result from jsondiff (backport #23795) (#23800)

This is an automatic backport of pull request #23795 done by [Mergify](https://mergify.com).


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Author: mergify[bot]

packages/@aws-cdk/cfnspec/build-tools/spec-diff.ts

@@ -237,6 +237,10 @@ async function main() {
     if (Array.isArray(update)) {
       changes.push(`* ${namespace} ${prefix} (__changed__)`);
       for (const entry of update) {
+        if (entry.length === 1 && entry[0] === ' ') {
+          // This means that this element of the array is unchanged
+          continue;
+        }
         if (entry.length !== 2) {
           throw new Error(`Unexpected array diff entry: ${JSON.stringify(entry)}`);
         }
@@ -247,7 +251,7 @@ async function main() {
           case '-':
             throw new Error(`Something awkward happened: ${entry[1]} was deleted from ${namespace} ${prefix}!`);
           case ' ':
-          // This entry is "context"
+            // This entry is "context"
             break;
           default:
             throw new Error(`Unexpected array diff entry: ${JSON.stringify(entry)}`);