fix: handle distribution.files being None (#6877)

We discovered that in some unknown situations, `distribution.files` can
be `None`, causing the assertion to fail. To handle `distribution.files`
being `None` more gracefully, we treat it as if it were an empty list.

Resolves: #6788 

src/poetry/utils/env.py

@@ -328,8 +328,8 @@ class SitePackages:
         for distribution in self.distributions(
             name=distribution_name, writable_only=writable_only
         ):
-            assert distribution.files is not None
-            for file in distribution.files:
+            files = [] if distribution.files is None else distribution.files
+            for file in files:
                 if file.name.endswith(suffix):
                     yield Path(
                         distribution.locate_file(file),  # type: ignore[no-untyped-call]
@@ -341,8 +341,8 @@ class SitePackages:
         for distribution in self.distributions(
             name=distribution_name, writable_only=writable_only
         ):
-            assert distribution.files is not None
-            for file in distribution.files:
+            files = [] if distribution.files is None else distribution.files
+            for file in files:
                 if file.name == name:
                     yield Path(
                         distribution.locate_file(file),  # type: ignore[no-untyped-call]
@@ -372,8 +372,8 @@ class SitePackages:
         for distribution in self.distributions(
             name=distribution_name, writable_only=True
         ):
-            assert distribution.files is not None
-            for file in distribution.files:
+            files = [] if distribution.files is None else distribution.files
+            for file in files:
                 path = Path(
                     distribution.locate_file(file),  # type: ignore[no-untyped-call]
                 )