[3.10] bpo-45183: don't raise an exception when calling zipimport.zipimporter.find_spec() when the zip file is missing and the internal cache has been reset (GH-28435) (#28438)

This can occur when the zip file gets deleted, you call zipimport.zipimporter.invalidate_cache(), and then try to use zipimport.zipimporter.find_spec() (i.e. you left the zip file path on sys.path).
(cherry picked from commit 209b7035f714dcc41df054b0b023e0b955d7e1a2)

Co-authored-by: Brett Cannon <brett@python.org>
diff --git a/Lib/zipimport.py b/Lib/zipimport.py
index c55fec6..25eaee9 100644
--- a/Lib/zipimport.py
+++ b/Lib/zipimport.py
@@ -334,7 +334,7 @@ def invalidate_caches(self):
             _zip_directory_cache[self.archive] = self._files
         except ZipImportError:
             _zip_directory_cache.pop(self.archive, None)
-            self._files = None
+            self._files = {}
 
 
     def __repr__(self):