Fix solc_standard_json regression on Windows - #191

There's another regression with crytic-compile due to the change in this commit:

It invokes solc --version but passes a blank dict for the env. Doing so will cause the following error on Windows:

  File "c:\users\x\documents\github\crytic-compile\crytic_compile\platform\solc.py", line 309, in get_version
    with subprocess.Popen(
  File "C:\Python\Python39\lib\subprocess.py", line 947, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "C:\Python\Python39\lib\subprocess.py", line 1416, in _execute_child
    hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
OSError: [WinError 87] The parameter is incorrect

This PR instead changes the blank dictionary to None, which will use the existing os.environ. Although disjointed behavior exists between how every compilation platform handles optional env variables and the logic should likely be cleaned up as well.

This was not tested on Linux/macOS, although None is already used as env for other platform invocations and is none problematic so this should be fine.