Investigate whether list of environment variables should be empty at runtime startup

[loganek]

The issue was raised in the comment: https://github.com/WebAssembly/wasi-testsuite/pull/74#discussion_r1189368447 If the list must not be empty, we should update related tests and spec documentation.

[sunfishcode]

It's valuable to be able to completely control the environment a WASI program runs in, so I propose WASI require the ability to run with an empty environment.