Fix handling an empty-body request with a required body param

[tiangolo]

🐛 Fix handling an empty-body request with a required body param, and add tests for it.

[codecov]

Codecov Report

Merging #311 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #311   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         222    224    +2     
  Lines        5273   5303   +30     
=====================================
+ Hits         5273   5303   +30

Impacted Files	Coverage Δ
fastapi/dependencies/utils.py	100% <100%> (ø)	⬆️
...rial/test_body_multiple_params/test_tutorial003.py	100% <100%> (ø)
docs/src/body_multiple_params/tutorial003.py	100% <0%> (ø)

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