fix(ts): encode undefined option args as null

[abhinavgautam01]

Fixes #1637

Summary

• Normalizes undefined instruction args to null when the IDL type is option.
• Prevents buffer-layout from skipping undefined optional fields during instruction encoding.
• Handles nested option fields inside defined structs/enums as well.
• Keeps non-option fields unchanged.

Testing

• yarn prettier src/coder/borsh/instruction.ts tests/coder-instructions.spec.ts --check
• yarn build:node
• yarn jest tests/coder-instructions.spec.ts --runInBand
• yarn test --runInBand
• git diff --check