Properly handle empty dictionary in get_range_spectrogram (#407)

* properly handle empty dictionary

* remove extra line

* fix lint

---------

Co-authored-by: Iara Ota <iara.ota@ligo.org>

gwsumm/data/range.py

@@ -122,16 +122,20 @@ def get_range_spectrogram(channel, segments, config=None, cache=None,
 
     # return correct data, according to state segment
     out = SpectrogramList()
-    for specgram in globalv.SPECTROGRAMS[key]:
-        for seg in segments:
-            if abs(seg) < specgram.dt.value:
-                continue
-            if specgram.span.intersects(seg):
-                common = specgram.span & type(seg)(seg[0],
-                                                   seg[1] + specgram.dt.value)
-                s = specgram.crop(*common)
-                if s.shape[0]:
-                    out.append(s)
+    try:
+        for specgram in globalv.SPECTROGRAMS[key]:
+            for seg in segments:
+                if abs(seg) < specgram.dt.value:
+                    continue
+                if specgram.span.intersects(seg):
+                    common = specgram.span & type(seg)(
+                        seg[0], seg[1] + specgram.dt.value)
+                    s = specgram.crop(*common)
+                    if s.shape[0]:
+                        out.append(s)
+    except KeyError:
+        # in case the key does not exist return empty SpectrogramList
+        return out
     return out.coalesce()