serialization: avoid empty additionalProperties

[yugangw-msft]

For https://github.com/Azure/azure-cli/issues/5733 I thought more about pruning at CLI level, but it has perf related drawback of having to go recursively to the leaf level; also, commands can create its own dictionary typed output which includes model instances, so CLI can't just check the root level types. So submit this PR to get conversation going. I am fine with any other options as long as we can mitigate the confusion. Json output is supposed to honestly reflect what is on the wire, and i hope the spirit can continue. //CC: @lmazuel @tjprescott @derekbekoe

[codecov-io]

Codecov Report

Merging #89 into master will increase coverage by 0.15%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
+ Coverage   28.94%   29.09%   +0.15%     
==========================================
  Files         380      380              
  Lines       16332    16170     -162     
==========================================
- Hits         4727     4705      -22     
+ Misses      11605    11465     -140

Impacted Files	Coverage Δ
msrest/serialization.py	82.29% <100%> (-0.03%)	:arrow_down:
test/vanilla/AcceptanceTests/test_url.py	22.72% <0%> (+0.17%)	:arrow_up:
...t/vanilla/AcceptanceTests/test_model_flattening.py	21.66% <0%> (+1.29%)	:arrow_up:
test/vanilla/AcceptanceTests/test_dictionary.py	13.04% <0%> (+1.88%)	:arrow_up:
.../vanilla/AcceptanceTests/test_required_optional.py	31.25% <0%> (+3.97%)	:arrow_up:

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[yugangw-msft]

@lmazuel, i am closing this out since you are not convinced this is the right thing to do