Add check for empty IFC in filtering

The ``files_filtered()`` method chokes on a ``'NoneType' object is not
subscriptable`` error if the ImageFileCollection is already empty before being
asked to filter further.

This commit adds a check for empty IFC and simply passes an empty list for the
set of files in the collection.

	modified:   CHANGES.rst
	modified:   ccdproc/image_collection.py

CHANGES.rst

@@ -1,3 +1,18 @@
+2.4.1 (unreleased)
+------------------
+
+New Features
+^^^^^^^^^^^^
+
+Other Changes and Additions
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Bug Fixes
+^^^^^^^^^
+
+- Fixes a crash when attempting to filter an already-empty ImageFileCollection,
+  instead simply returning an empty ImageFileCollection.  [#801]
+
 2.4.0 (2022-11-16)
 ------------------
 

ccdproc/image_collection.py

@@ -366,6 +366,10 @@ def files_filtered(self, **kwd):
         Value comparison is case *insensitive* for strings, whether matching
         exactly or matching with regular expressions.
         """
+        # If the collection is empty, self.summary == None; return empty list
+        if self.summary is None:
+            return []
+
         # force a copy by explicitly converting to a list
         current_file_mask = self.summary['file'].mask.tolist()