Validate route cannot be the empty string

This results in an opaque error message when you try to deploy.

chalice/deploy/deployer.py

@@ -89,6 +89,8 @@ def validate_routes(routes):
     #
     # * any routes that end with a trailing slash.
     for route_name, methods in routes.items():
+        if not route_name:
+            raise ValueError("Route cannot be the empty string")
         if route_name != '/' and route_name.endswith('/'):
             raise ValueError("Route cannot end with a trailing slash: %s"
                              % route_name)

tests/unit/deploy/test_deployer.py

@@ -315,6 +315,14 @@ def test_trailing_slash_routes_result_in_error():
         validate_configuration(config)
 
 
+def test_empty_route_results_in_error():
+    app = Chalice('appname')
+    app.routes = {'': {}}
+    config = Config.create(chalice_app=app)
+    with pytest.raises(ValueError):
+        validate_configuration(config)
+
+
 def test_validate_python_version_invalid():
     config = mock.Mock(spec=Config)
     config.lambda_python_version = 'python1.0'