Do not cache unimportable compiled module (#1079)

From the issue:

The issue can be reproduced by getting the description of the QtBluetooth module from PyQt5 on Windows:

import jedi
completions = jedi.Script('import PyQt5.QtBlueTooth').completions()
completions[0].description

It's hard to write a test for it so we don't write one for it.

jedi/evaluate/imports.py

@@ -500,12 +500,14 @@ def _load_module(evaluator, path=None, code=None, sys_path=None,
             )
         else:
             module = compiled.load_module(evaluator, path=path, sys_path=sys_path)
+
+    if module_name is not None and module is not None:
         add_module(evaluator, module_name, module, safe=safe_module_name)
+
     return module
 
 
 def add_module(evaluator, module_name, module, safe=False):
-    if module_name is not None:
     if not safe and '.' not in module_name:
         # We cannot add paths with dots, because that would collide with
         # the sepatator dots for nested packages. Therefore we return