Improve timerange parsing when accepting values from API

freqtrade/configuration/timerange.py

@@ -116,7 +116,7 @@ class TimeRange:
         :param text: value from --timerange
         :return: Start and End range period
         """
-        if text is None:
+        if not text:
             return TimeRange(None, None, 0, 0)
         syntax = [(r'^-(\d{8})$', (None, 'date')),
                   (r'^(\d{8})-$', ('date', None)),

tests/test_timerange.py

@@ -10,6 +10,8 @@ from freqtrade.exceptions import OperationalException
 
 def test_parse_timerange_incorrect():
 
+    timerange = TimeRange.parse_timerange('')
+    assert timerange == TimeRange(None, None, 0, 0)
     timerange = TimeRange.parse_timerange('20100522-')
     assert TimeRange('date', None, 1274486400, 0) == timerange
     assert timerange.timerange_str == '20100522-'