fix(A1Z26): return empty string instead of empty array for empty input

The A1Z26 Cipher Decode operation declares outputType as 'string' but
returned an empty array for empty input, violating its type contract.

Fixes #2248

src/core/operations/A1Z26CipherDecode.mjs

@@ -76,7 +76,7 @@ class A1Z26CipherDecode extends Operation {
         const delim = Utils.charRep(args[0] || "Space");
 
         if (input.length === 0) {
-            return [];
+            return "";
         }
 
         const bites = input.split(delim);