[core/theme] State == "" causes backtraces

Empty states ("" rather than None) caused a backtrace in the theme
engine. Fix that by being a bit more robust, and add a test that checks
for regressions.

fixes #406

bumblebee/theme.py

@@ -241,7 +241,8 @@ class Theme(object):
         states = widget.state()
         if name not in states:
             for state in states:
-                state_themes.append(self._get(widget, state, {}))
+                if state:
+                    state_themes.append(self._get(widget, state, {}))
 
         value = self._defaults.get(name, default)
         value = widget.get("theme.{}".format(name), value)

tests/test_theme.py

@@ -118,6 +118,14 @@ class TestTheme(unittest.TestCase):
         # widget theme instead (i.e. no fallback to a more general state theme)
         self.assertEquals(theme.bg(self.themedWidget), data[self.widgetTheme]["bg"])
 
+    def test_empty_state(self):
+        theme = self.theme
+        data = theme.data()
+
+        self.anyModule.state.return_value = ""
+        self.assertEquals(theme.fg(self.anyWidget), data["defaults"]["fg"])
+        self.assertEquals(theme.bg(self.anyWidget), data["defaults"]["bg"])
+
     def test_separator(self):
         self.assertEquals(self.validThemeSeparator, self.theme.separator(self.anyWidget))