emsdk

Fix fatal exception in "emsdk list" on unsupported platforms (#549)

On platforms with no supported binary SDKs to download, find_sdk
can return None which caused a check in the "list" command to fail.
Checking for the return value before making method calls fixes this,
and allows Linux/ARM64 to run "emsdk list" and then build an SDK
from source.

Fixes https://github.com/emscripten-core/emsdk/issues/548