[Fix] Fix potential bug about output_config overwrite

[CarolineCheng233]

This PR fixes potential bug when users do not assign parser args. For example, if a user does not assign args.out

output_config = dict(out=f'{work_dir}/results.json', output_format='json')
args.out = None
output_config = Config._merge_a_into_b(dict(out=args.out), output_config)

then, output_config in config files will be overwritten.

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[codecov]

Codecov Report

Merging #463 (b13224d) into master (137351c) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #463      +/-   ##
==========================================
+ Coverage   84.67%   84.71%   +0.03%     
==========================================
  Files         118      118              
  Lines        8347     8347              
  Branches     1366     1366              
==========================================
+ Hits         7068     7071       +3     
+ Misses        932      931       -1     
+ Partials      347      345       -2     

Flag	Coverage Δ
unittests	84.70% <ø> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmaction/core/evaluation/accuracy.py	94.52% <0.00%> (+1.36%)	⬆️

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[innerlee]

Thanks! This bug was introduced in #310, in which we replaced our own merge function with mmcv's.

In our version of merge, we treat None as transparent, and would not overwrite things.
It was my oversight that, I wrongly thought mmcv's merge would respect this semantic.

This has caused many problems. Now I'm thinking whether we should bring our merge function back. mmcv's functions sometimes are less thoughtful :/