Add check for empty IFC in filtering

[tbowers7]

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 an empty IFC and simply passes an empty list for the set of files in the collection.

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

Please have a look at the following list and replace the "[ ]" with a "[x]" if
the answer to this question is yes.

• For new contributors: Did you add yourself to the "Authors.rst" file?

For documentation changes:

• For documentation changes: Does your commit message include a "[skip ci]"?
  Note that it should not if you changed any examples!

For bugfixes:

• Did you add an entry to the "Changes.rst" file?
• Did you add a regression test?
• Does the commit message include a "Fixes #issue_number" (replace "issue_number").
• Does this PR add, rename, move or remove any existing functions or parameters?

For new functionality:

• Did you add an entry to the "Changes.rst" file?
• Did you include a meaningful docstring with Parameters, Returns and Examples?
• Does the commit message include a "Fixes #issue_number" (replace "issue_number").
• Did you include tests for the new functionality?
• Does this PR add, rename, move or remove any existing functions or parameters?

Please note that the last point is not a requirement. It is meant as a check if
the pull request potentially breaks backwards-compatibility.

---

[mwcraig]

CLosing/reopening to re-run CI

[codecov]

Codecov Report

Merging #801 (b02f2e1) into main (623162d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #801   +/-   ##
=======================================
  Coverage   97.55%   97.55%           
=======================================
  Files           9        9           
  Lines        1390     1392    +2     
=======================================
+ Hits         1356     1358    +2     
  Misses         34       34           

Impacted Files	Coverage Δ
ccdproc/image_collection.py	99.20% <100.00%> (+<0.01%)	⬆️

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