Fix action handling for no contents

[relu91]

This PR tackles both problems described in #1041. In particular, now the HTTP server responds using 204 when no content is provided and the payload contains the number zero for the testing ( see 1e7b3fc).

Fix #1041

[JKRhb]

Thank you, @relu91, the changes look good to me :) See a couple of minor suggestions/comments below.

[codecov-commenter]

Codecov Report

Merging #1042 (bc6030c) into master (567e979) will increase coverage by 0.37%.
Report is 6 commits behind head on master.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #1042      +/-   ##
==========================================
+ Coverage   75.43%   75.80%   +0.37%     
==========================================
  Files          80       80              
  Lines       15185    15303     +118     
  Branches     1434     1444      +10     
==========================================
+ Hits        11455    11601     +146     
+ Misses       3697     3669      -28     
  Partials       33       33              

Files Changed	Coverage Δ
packages/binding-http/src/routes/action.ts	58.58% <100.00%> (+4.04%)	⬆️
packages/core/src/exposed-thing.ts	66.91% <100.00%> (+0.14%)	⬆️

... and 3 files with indirect coverage changes

[danielpeintner]

PR looks good.

Very minor comment below that is up to you ...