Fix traffic display for SR discovery promos

The "Read from PromoCampaign things" change broke the code that causes the
traffic display to show the past month by default if a promoted link has
no campaigns.

This change fixes a bug where clicking on "traffic" on one of the subreddit
discovery spotlight links caused a 500 error.

See: 273c548

r2/r2/lib/promote.py

@@ -970,12 +970,6 @@ def get_total_run(link):
 
     campaigns = PromoCampaign._by_link(link._id)
 
-    # a manually launched promo (e.g., sr discovery) might not have campaigns.
-    if not campaigns: 
-        latest = datetime.utcnow()
-        earliest = latest - timedelta(days=30)  # last month
-        return earliest, latest
-
     earliest = None
     latest = None
     for campaign in campaigns:
@@ -987,7 +981,12 @@ def get_total_run(link):
 
         if not latest or campaign.end_date > latest:
             latest = campaign.end_date
-
+
+    # a manually launched promo (e.g., sr discovery) might not have campaigns.
+    if not earliest or not latest: 
+        latest = datetime.utcnow()
+        earliest = latest - timedelta(days=30)  # last month
+
     # ugh this stuff is a mess. they're stored as "UTC" but actually mean UTC-5.
     earliest = earliest.replace(tzinfo=None) - timezone_offset
     latest = latest.replace(tzinfo=None) - timezone_offset