idl: Disallow empty discriminators

[acheroncrypto]

Problem

It's possible to set the discriminators to an empty array. For example:

#[account(discriminator = [])]
pub struct MyAccount {}

This shouldn't be allowed due to security and convenience reasons.

Summary of changes

Disallow empty discriminators during IDL generation.

The reason for handling this during IDL build process is because we can't reliably get the discriminator from parsing the input (e.g. discriminator = EMPTY is also valid).

And the reason for not implementing our own custom discriminator type e.g.

pub struct NonZeroDiscriminator(&'static [u8]);

is because it would not allow using programs that don't have discriminators (e.g. some of the SPL programs).

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Note: This PR is part of a greater effort explained in #3097.

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