Avoid adding None loss values in training_epoch_end

[carmocca]

What does this PR do?

In manual optimization, users are not forced to return a loss in their training_step.
However, this meant that a None loss was getting added in training_epoch_end.

Also reduced code duplication in the tests.

Part of #7631

Even though this is a bug-fix, it doesn't need to go into 1.3.x as no user reported this.

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[codecov]

Codecov Report

Merging #7772 (a2fa7c0) into master (a69beab) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #7772   +/-   ##
======================================
- Coverage      92%     92%   -0%     
======================================
  Files         199     199           
  Lines       12986   12987    +1     
======================================
- Hits        11994   11982   -12     
- Misses        992    1005   +13