Correct handling of empty/undef in list values

- Expansion of a list including `None` values, violating 3.2.1 as the
  undefined value is not ignored, e.g.

  >>> expand('/test{/x*}', x=['one', None, 'three'])
  '/test/one/None/three'

- A list with empty string was ignored, violating 2.3 as this should not
  be tread as undefined, example:

  >>> expand('/test{/x}', x=[])
  '/test'
  >>> expand('/test{/x}', x='')
  '/test/'
  >>> expand('/test{/x}', x=[''])
  '/test'  # should be /test/
  >>> expand('/test{/x}', x=['', ''])
  '/test/,'
  >>> expand('/test{/x*}', x=['', ''])
  '/test//'
  >>> expand('/test{/x*}', x=[''])
  '/test'

2 files changed, 14 insertions, 4 deletions

diff --git a/tests/test_uritemplate.py b/tests/test_uritemplate.py
index 9977ef7..f67ad26 100644
--- a/tests/test_uritemplate.py
+++ b/tests/test_uritemplate.py
@@ -470,12 +470,24 @@ class TestURITemplate(RFCTemplateExamples('RFCMeta', (TestCase,), {})):
             'foo', [], True, '/'), None
         )
         self.assertEqual(t.variables[0]._label_path_expansion(
+            'foo', [None], True, '/'), None
+        )
+        self.assertEqual(t.variables[0]._label_path_expansion(
+            'foo', [None, None], True, '/'), None
+        )
+        self.assertEqual(t.variables[0]._label_path_expansion(
             'foo', ['one'], True, '/'), 'one'
         )
         self.assertEqual(t.variables[0]._label_path_expansion(
             'foo', ['one', 'two'], True, '/'), 'one/two'
         )
         self.assertEqual(t.variables[0]._label_path_expansion(
+            'foo', ['one', None, 'two'], True, '/'), 'one/two'
+        )
+        self.assertEqual(t.variables[0]._label_path_expansion(
+            'foo', [''], True, '/'), ''
+        )
+        self.assertEqual(t.variables[0]._label_path_expansion(
             'foo', ['', ''], True, '/'), '/'
         )
 
diff --git a/uritemplate/variable.py b/uritemplate/variable.py
index a3bd4ce..ce3f652 100644
--- a/uritemplate/variable.py
+++ b/uritemplate/variable.py
@@ -200,10 +200,8 @@ class URIVariable(object):
             if not explode:
                 join_str = ','
 
-            expanded = join_str.join(
-                quote(v, safe) for v in value if value is not None
-            )
-            return expanded if expanded else None
+            fragments = [quote(v, safe) for v in value if v is not None]
+            return join_str.join(fragments) if fragments else None
 
         if dict_test(value) or tuples:
             items = items or sorted(value.items())