ts: Support empty account structs

[2501babe]

i may be a usecase of one, but if you declare a struct like

#[account]
#[derive(Debug, Default)]
pub struct Post {}

then the program builds and deploys fine, idl outputs fine, but when you load the idl in js it does this

/home/hana/work/hana/circuit/scratch/node_modules/@project-serum/anchor/dist/cjs/coder/common.js:32
        .reduce((a, b) => a + b);
         ^

TypeError: Reduce of empty array with no initial value
    at Array.reduce (<anonymous>)
    at Object.accountSize (/home/hana/work/hana/circuit/scratch/node_modules/@project-serum/anchor/dist/cjs/coder/common.js:32:10)
    at new AccountClient (/home/hana/work/hana/circuit/scratch/node_modules/@project-serum/anchor/dist/cjs/program/namespace/account.js:51:67)
    at /home/hana/work/hana/circuit/scratch/node_modules/@project-serum/anchor/dist/cjs/program/namespace/account.js:39:32
    at Array.forEach (<anonymous>)
    at Function.build (/home/hana/work/hana/circuit/scratch/node_modules/@project-serum/anchor/dist/cjs/program/namespace/account.js:37:22)
    at Function.build (/home/hana/work/hana/circuit/scratch/node_modules/@project-serum/anchor/dist/cjs/program/namespace/index.js:43:33)
    at new Program (/home/hana/work/hana/circuit/scratch/node_modules/@project-serum/anchor/dist/cjs/program/index.js:57:96)
    at Object.<anonymous> (/home/hana/work/hana/circuit/scratch/index.js:14:17)
    at Module._compile (node:internal/modules/cjs/loader:1109:14)