Fix an issue where the seed is not used when it is 0

[Manchewable]

What does this PR do?

This PR fix allows us to use the value 0 as the seed. Currently, when the seed is configured as 0 in the config file, the seed will not be used in train.py as it will not bypass the if cfg.get("seed"): check. By checking whether cfg.get("seed") is the None type, it'll let us set the seed to 0.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.24%. Comparing base (bddbc24) to head (1e3bdcf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #635   +/-   ##
=======================================
  Coverage   83.24%   83.24%           
=======================================
  Files          11       11           
  Lines         376      376           
=======================================
  Hits          313      313           
  Misses         63       63           

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