Fix: Missing discriminator for instructions without parameters

[t0mpl]

Problem:

Instructions with no parameters were being generated with nil instruction data instead of including their 8-byte discriminator. This caused InstructionFallbackNotFound errors when calling these instructions on-chain.
The generator was only creating the buffer and writing the discriminator when len(instruction.Args) > 0. Instructions like cancel_order, force_void, initialize_market_factory, etc. would fail because Anchor always expects the discriminator, even for parameter-less instructions.

Solution:

Refactored the instruction generation logic to always:
Create the buffer and encoder
Write the instruction discriminator
Encode parameters only when they exist
The buffer is now always passed to NewInstruction() instead of conditionally passing nil.

Changes:

Modified generator/instructions.go to move discriminator encoding outside the parameter check
All instructions now include their discriminator in the instruction data
Parameter encoding remains conditional