Enforce that config.String gets a value

This makes it more consistent with the other config parsers. If the
string value should truly be optional, the user should explicitly mark
it as such by wrapping it with config.Optional.

baseplate/config.py

@@ -75,6 +75,8 @@ def __str__(self):  # pragma: nocover
 
 def String(text):
     """A raw string."""
+    if not text:
+        raise ValueError("no value specified")
     return text
 
 

tests/unit/config_tests.py

@@ -14,6 +14,10 @@ def test_parse_string(self):
         result = config.String("whatever")
         self.assertEqual(result, "whatever")
 
+    def test_empty_string_not_ok(self):
+        with self.assertRaises(ValueError):
+            config.String("")
+
 
 class IntegerTests(unittest.TestCase):
     def test_parse_integer_valid(self):
@@ -189,7 +193,7 @@ def test_simple_config(self):
             },
 
             "noo": {
-                "bar": config.String,
+                "bar": config.Optional(config.String, default=""),
             },
 
             "deep": {