Environment keys can contain empty values

Environment keys that contain no value, get populated with values taken
from the environment not from the build phase but from running the command `up`.

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

tests/unit/config/config_test.py

@@ -414,6 +414,23 @@ class InterpolationTest(unittest.TestCase):
         self.assertIn('in service "web"', cm.exception.msg)
         self.assertIn('"${"', cm.exception.msg)
 
+    def test_empty_environment_key_allowed(self):
+        service_dict = config.load(
+            build_config_details(
+                {
+                    'web': {
+                        'build': '.',
+                        'environment': {
+                            'POSTGRES_PASSWORD': ''
+                        },
+                    },
+                },
+                '.',
+                None,
+            )
+        )[0]
+        self.assertEquals(service_dict['environment']['POSTGRES_PASSWORD'], '')
+
 
 class VolumeConfigTest(unittest.TestCase):
     def test_no_binding(self):