timetracking: Handle no data available for the requested month

Not emitting any record if no data is availablre for the requested
month seems more useful than failing with an exception.

timetracking.py

@@ -123,7 +123,7 @@ class Client:
                     data['uiDdlMonth'] = value
                     break
             else:
-                raise ValueError
+                return None
 
             # POST ReportFunction.aspx
             r = self.session.post(r.url, data=data)
@@ -399,6 +399,8 @@ def timesheet(client, month, n, v):
 
     for month in months:
         data = client.timesheet(month)
+        if not data:
+            continue
         timesheet = Timesheet(month, data)
 
         for day, booking in timesheet: