Support vars with `null/none` values
dbt thinks that variables with `null` values are not set. We should distinguish between

1) not set
2) set to `none`
diff --git a/core/dbt/context/common.py b/core/dbt/context/common.py
index 6361d675a..59aa97743 100644
--- a/core/dbt/context/common.py
+++ b/core/dbt/context/common.py
@@ -206,8 +206,7 @@ def log(msg, info=False):
 class Var(object):
     UndefinedVarError = "Required var '{}' not found in config:\\nVars "\\
                         "supplied to {} = {}"
-    NoneVarError = "Supplied var '{}' is undefined in config:\\nVars supplied "\\
-                   "to {} = {}"
+    _VAR_NOTSET = object()
 
     def __init__(self, model, context, overrides):
         self.model = model
@@ -241,7 +240,7 @@ class Var(object):
         return json.dumps(data, sort_keys=True, indent=4)
 
     def assert_var_defined(self, var_name, default):
-        if var_name not in self.local_vars and default is None:
+        if var_name not in self.local_vars and default is self._VAR_NOTSET:
             pretty_vars = self.pretty_dict(self.local_vars)
             dbt.exceptions.raise_compiler_error(
                 self.UndefinedVarError.format(
@@ -250,25 +249,12 @@ class Var(object):
                 self.model
             )
 
-    def assert_var_not_none(self, var_name):
-        raw = self.local_vars[var_name]
-        if raw is None:
-            pretty_vars = self.pretty_dict(self.local_vars)
-            dbt.exceptions.raise_compiler_error(
-                self.NoneVarError.format(
-                    var_name, self.model_name, pretty_vars
-                ),
-                self.model
-            )
-
-    def __call__(self, var_name, default=None):
+    def __call__(self, var_name, default=_VAR_NOTSET):
         self.assert_var_defined(var_name, default)
 
         if var_name not in self.local_vars:
             return default
 
-        self.assert_var_not_none(var_name)
-
         raw = self.local_vars[var_name]
 
         # if bool/int/float/etc are passed in, don't compile anything
diff --git a/test/unit/test_context.py b/test/unit/test_context.py
index c5d9a5c99..bf286dc79 100644
--- a/test/unit/test_context.py
+++ b/test/unit/test_context.py
@@ -43,14 +43,19 @@ class TestVar(unittest.TestCase):
         )
         self.context = mock.MagicMock()
 
-    def test_var_not_none_is_none(self):
+    def test_var_default_something(self):
+        var = Var(self.model, self.context, overrides={'foo': 'baz'})
+        self.assertEqual(var('foo'), 'baz')
+        self.assertEqual(var('foo', 'bar'), 'baz')
+
+    def test_var_default_none(self):
         var = Var(self.model, self.context, overrides={'foo': None})
-        var.assert_var_defined('foo', None)
-        with self.assertRaises(dbt.exceptions.CompilationException):
-            var.assert_var_not_none('foo')
+        self.assertEqual(var('foo'), None)
+        self.assertEqual(var('foo', 'bar'), None)
 
-    def test_var_defined_is_missing(self):
+    def test_var_not_defined(self):
         var = Var(self.model, self.context, overrides={})
-        var.assert_var_defined('foo', 'bar')
+
+        self.assertEqual(var('foo', 'bar'), 'bar')
         with self.assertRaises(dbt.exceptions.CompilationException):
-            var.assert_var_defined('foo', None)
+            var('foo')
['test/unit/test_context.py', 'core/dbt/context/common.py']