Fix uncaught TypeError in To BCD with empty encoding scheme

[itxaiohanglover]

Description

The To BCD operation throws an uncaught TypeError when its Scheme
argument is empty or unrecognised. This can be triggered via a recipe URL where
the first argument is an empty string, e.g.:

https://gchq.github.io/CyberChef/#recipe=To_BCD('',true,false,'Nibbles')&input=NDM

To BCD - TypeError in blob:... on line 2.
Message: can't access property 4, n is undefined

Root cause

In src/core/operations/ToBCD.mjs, the encoding lookup is performed with:

const encoding = ENCODING_LOOKUP[args[0]];

args[0] is the Scheme option. When it is an empty string (or any value not
present in ENCODING_LOOKUP), encoding is undefined. The code then does
encoding[n] while mapping over the input digits, which throws the uncaught
TypeError because a property is being accessed on undefined.

From BCD (src/core/operations/FromBCD.mjs) shares the same pattern: it calls
encoding.indexOf(n), which would throw TypeError: encoding.indexOf is not a function for the same reason.

The solution

Both operations now validate the encoding scheme immediately after the lookup and
throw a friendly OperationError instead of letting an uncaught TypeError
crash the bake:

if (encoding === undefined)
    throw new OperationError("Invalid encoding scheme");

This mirrors the existing validation style already used in To BCD (e.g. the
input.isNaN() and fractional-value guards) and the approach taken by other
recent fixes that convert uncaught TypeErrors into actionable errors. As with
those existing OperationErrors, the message is surfaced to the user as the
operation output rather than crashing the bake.

Existing Issue

Fixes #2488

Screenshots

No visual/UI changes — this only replaces a crash with a handled error message.

AI disclosure

This code was written with the assistance of an AI coding assistant (Qoder). The
logic, fix, and tests have been reviewed and verified against the existing
codebase conventions and the Recipe.execute error-handling flow.

Test Coverage

Added regression tests to tests/operations/tests/BCD.mjs covering both
operations with an empty scheme:

• To BCD: invalid (empty) encoding scheme — input 43, expects output
  Invalid encoding scheme.
• From BCD: invalid (empty) encoding scheme — input 0100 0011, expects
  output Invalid encoding scheme.

Because an OperationError is caught by Recipe.execute and its message is
returned as the operation output, these tests assert the message directly
(expectedOutput: "Invalid encoding scheme"). Without the fix, both cases
produce an uncaught TypeError instead of this message, so the tests fail
without the fix and pass with it.

[CLAassistant]

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