Fix missing default input file value in the cli

[NyanKiyoshi]

Fixes #15.

Changes Checklist

• The changes were tested (manually)
• The changes are tested automatically (pytest)
• The changes are optimized and clean
• The changes are documented:
  • The code is documented
  • The readme is up to date
  • The documentation (readthedocs) is up to date and tested

[codecov-io]

Codecov Report

Merging #17 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #17   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         128    130    +2     
  Branches       13     13           
=====================================
+ Hits          128    130    +2

Impacted Files	Coverage Δ
pytest_django_queries/plugin.py	100% <100%> (ø)	⬆️
pytest_django_queries/cli.py	100% <100%> (ø)	⬆️

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