Fix a TypeError when requires_dist is null on PyPI

CHANGELOG.md

 # Change Log
 
+
+## [Unreleased]
+
+### Fixed
+
+- Fixed TypeError when `requires_dist` is null on PyPI.
+
+
 ## [0.4.1] - 2018-03-08
 
 ### Fixed

poetry/repositories/pypi_repository.py

@@ -83,7 +83,8 @@ class PyPiRepository(Repository):
 
             release_info = self.get_release_info(name, version)
             package = Package(name, version, version)
-            for req in release_info['requires_dist']:
+            requires_dist = release_info['requires_dist'] or []
+            for req in requires_dist:
                 try:
                     req = InstallRequirement.from_line(req)
                 except Exception: