Fix issue pulling future NFL boxscores

For NFL games that are scheduled but have not yet taken place, the code
would take the preview link for the game and treat it as the Boxscore
link, causing errors to be thrown while attempt to pull Boxscore data
from the preview which doesn't have the intended information.

Signed-Off-By: Robert Clark <robdclark@outlook.com>

sportsreference/nfl/boxscore.py

@@ -729,6 +729,8 @@ def _alt_abbreviations(self, boxscore):
         for column in game_info('th').items():
             if column.text():
                 abbreviations.append(column.text())
+        if not abbreviations:
+            return None, None
         return abbreviations
 
     def _parse_game_data(self, uri):

tests/unit/test_nfl_boxscore.py

@@ -336,6 +336,12 @@ def test_finding_away_team_with_no_abbrs(self):
 
         assert team == AWAY
 
+    def test_missing_abbreviations(self):
+        table = '<table id="team_stats"><thead></thead></table>'
+        output = self.boxscore._alt_abbreviations(pq(table))
+
+        assert output == (None, None)
+
 
 class TestNFLBoxscores:
     @patch('requests.get', side_effect=mock_pyquery)