return desired channels when no channels are found

derived.get_timeseries currently looks for channels present in a stream and compares them to desired channels. This becomes problematic when no channels are found as we are trying to iterate a NoneType in the set difference

test/DerivedTimeseriesFactory_test.py

@@ -2,8 +2,9 @@ from typing import List
 
 from obspy import Stream
 
-from geomagio import DerivedTimeseriesFactory, TimeseriesUtility
+from geomagio import TimeseriesUtility
 from geomagio.algorithm import Algorithm, DeltaFAlgorithm, XYZAlgorithm
+from geomagio.DerivedTimeseriesFactory import DerivedTimeseriesFactory, get_missing
 from geomagio.iaga2002 import StreamIAGA2002Factory
 from geomagio.edge import EdgeFactory
 
@@ -91,6 +92,17 @@ def test_get_timeseries():
     assert set(TimeseriesUtility.get_channels(timeseries)) == set(["H", "D"])
 
 
+def test_get_missing():
+    """test.DerivedTimeseriesFactory_test.test_get_missing()"""
+    desired = ["X", "Y", "D", "G"]
+    assert set(get_missing(input=Stream(), desired=desired)) == set(desired)
+    desired = ["H", "E", "Z", "F"]
+    timeseries = get_derived_timeseries(
+        "etc/filter/BOU20200101vsec.sec", desired, "variation", "second"
+    )
+    assert get_missing(input=timeseries, desired=desired) == []
+
+
 def get_derived_timeseries(
     url: str, channels: List[str], data_type: str, interval: str
 ) -> Stream: