Stop service uploaders crashing if no logged data

This only happens if live or hourly logging is run before any data has
been logged. See issue #64

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

src/pywws/__init__.py

@@ -1,3 +1,3 @@
 __version__ = '18.6.0'
-_release = '1546'
-_commit = '8705776'
+_release = '1547'
+_commit = '984c289'

src/pywws/service/__init__.py

@@ -137,8 +137,8 @@ def __init__(self, context):
             self.templater = pywws.template.Template(context, use_locale=False)
             self.template_file = StringIO(self.template)
         # set timestamp of first data to upload
-        earliest = self.context.calib_data.before(datetime.max) - max(
-            timedelta(days=self.catchup), self.interval)
+        earliest = self.context.calib_data.before(datetime.max) or datetime.max
+        earliest -= max(timedelta(days=self.catchup), self.interval)
         self.next_update = context.status.get_datetime(
             'last update', self.service_name)
         if self.next_update:

src/pywws/toservice.py

@@ -273,7 +273,8 @@ def __init__(self, context, service_name):
         # set timestamp of first data to upload
         self.next_update = datetime.utcnow() - max(
             timedelta(days=self.catchup), self.interval)
-        self.next_update = min(self.next_update, self.data.before(datetime.max))
+        self.next_update = min(
+            self.next_update, self.data.before(datetime.max) or datetime.max)
 
     def prepare_data(self, data):
         """Prepare a weather data record.