Discard empty strings when parsing font-family

src/tinycss/fonts3.py

@@ -15,7 +15,9 @@ def parse_font_family(css_string):
     families = []
     current_family = ''
     def commit():
-        families.append(current_family.strip())
+        val = current_family.strip()
+        if val:
+            families.append(val)
 
     for token in tokenize_grouped(css_string.strip()):
         if token.type == 'STRING':

src/tinycss/tests/fonts3.py

@@ -42,6 +42,7 @@ class TestFonts3(BaseTest):
                 r'"X \"Y"': ['X "Y'],
                 'A B, C D, "E", serif': ['A B', 'C D', 'E', 'serif'],
                 '': [],
+                '"", a': ['a'],
         }.iteritems():
             self.ae(q, parse_font_family(raw))
         for single in ('serif', 'sans-serif', 'A B C'):