Cope better when very little logged data

If you leap straight into processing with a new station there might not
be enough data to generate a monthly or daily file.

Signed-off-by: Jim Easterbrook <jim@jim-easterbrook.me.uk>

src/pywws/__init__.py

@@ -1,3 +1,3 @@
 __version__ = '18.8.0'
-_release = '1586'
-_commit = 'd2d51a1'
+_release = '1587'
+_commit = '40be4b4'

src/pywws/process.py

@@ -683,6 +683,8 @@ def generate_monthly(rain_day_threshold, day_end_hour, use_dst,
         start -= DAY
     del monthly_data[start:]
     stop = daily_data.before(datetime.max)
+    if stop is None:
+        return None
     month_start = start
     acc = MonthAcc(rain_day_threshold)
     count = 0