idl: Disallow all zero account discriminators

[acheroncrypto]

Problem

Setting account discriminators to all zeros conflicts with the zero account constraint (which checks the discriminator bytes to be all zeros). For example:

#[account(discriminator = 0)]
pub struct MyAccount {}

#[derive(Accounts)]
pub struct MyIx<'info> {
    #[account(zero)]
    pub my_account: Account<'info, MyAccount>,
}

Summary of changes

Return an error during IDL generation if an account has all zero discriminator.

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

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