Revert config unit tests to compare attributes to empty strings again

Reverts a change in commit 2eacb4e.

The comparison in this unit test compared an *attribute* to an empty
string, not a *string*. Furthermore, the entire point of the test was
to verify that an empty attribute doesn't compare equal to the empty
string. In its old form, the test was just wrong (and failing).

src/tests/test_config.cpp

@@ -149,13 +149,13 @@ BOOST_AUTO_TEST_CASE ( test_config_attribute_value )
 	x_str = c["x"].str();
 		BOOST_CHECK_EQUAL(x_str, "123456789123");
 
-		// blank != "" test.
+	// blank != "" test.
 	c.clear();
-	BOOST_CHECK(!cc["x"].empty());
+	BOOST_CHECK(cc["x"] != "");
 	BOOST_CHECK(cc["x"].empty());
 	BOOST_CHECK(cc["x"].blank());
 
-	BOOST_CHECK(!c["x"].empty());
+	BOOST_CHECK(c["x"] != "");
 	BOOST_CHECK(c["x"].empty());
 	BOOST_CHECK(c["x"].blank());