wast2json should not complain on an empty wast file

[yamt]

if my reading of the spec is correct, an empty wast file is valid.
https://github.com/WebAssembly/spec/blob/main/interpreter/README.md#scripts

an example of such a file:
https://github.com/WebAssembly/multi-memory/blob/4f6b8f53ec11e59f5e38033db4199db18df83706/test/core/multi-memory/memory_copy1.wast#L1

how other implementations behave:

spacetanuki% touch empty.wast
spacetanuki% wast2json empty.wast 
empty.wast:1:1: error: unexpected token "EOF", expected a module field or a command.
spacetanuki% ~/git/wasm/spec/interpreter/wasm empty.wast
spacetanuki% wasmtime wast empty.wast 
Error: failed to run script file 'empty.wast'

Caused by:
    expected at least one module field
         --> empty.wast:1:1
          |
        1 | 
          | ^
spacetanuki% wasmer wast empty.wast  
error: failed to test the wast `empty.wast`
│   1: tests failed
╰─▶ 2: expected at least one module field
            --> empty.wast:1:1
             |
           1 | 
             | ^
spacetanuki% 

[keithw]

We do this on purpose since #160 which predates the MVP (and it appears wasmtime and wasmer match us). But, I agree that empty modules/scripts are technically valid... Maybe we can make this a warning instead?