Handle modules with __file__ property being None

It's been discovered that some modules have the `__file__` property
being set as `None`. An example of a such module is `backports`.

Providing `None` to `os.path.abspath()` causes an exception:
`TypeError: expected str, bytes or os.PathLike object, not NoneType`

Related to https://github.com/mitogen-hq/mitogen/issues/946

mitogen/master.py

@@ -148,7 +148,11 @@ def is_stdlib_name(modname):
         return False
 
     # six installs crap with no __file__
-    modpath = os.path.abspath(getattr(module, '__file__', ''))
+    path = getattr(module, '__file__', '')
+    if path is None:
+        path = ''
+
+    modpath = os.path.abspath(path)
     return is_stdlib_path(modpath)