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

[brick-pixel]

What

Changed the early return for empty input in A1Z26 Cipher Decode from [] to "".

Why

The operation declares this.outputType = "string", but the empty input check on line 80 returns an empty array:

if (input.length === 0) {
    return [];  // violates outputType: "string"
}

This causes a type mismatch that can break downstream operations expecting a string.

Changes

File: src/core/operations/A1Z26CipherDecode.mjs

 if (input.length === 0) {
-    return [];
+    return "";
 }

Fixes #2248

[CLAassistant]

All committers have signed the CLA.

[GCHQDeveloper581]

Please could you add a test that would fail without this change and pass with it.

[brick-pixel]

Added tests in 06cc1b4:

• A1Z26 Cipher Decode: basic decode — verifies normal decoding (8 5 12 12 15 → hello)
• A1Z26 Cipher Decode: empty input returns empty string — this test fails without the fix (returns [] instead of "") and passes with it

Test file: tests/operations/tests/A1Z26CipherDecode.mjs
Import added to: tests/operations/index.mjs

[GCHQDeveloper581]

Thanks for your contribution!