fix(testing): parse response body to undefined if not provided in case of "application/json" header

[JozefFlakus]

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

• @marblejs/testing - parses response body to an empty string if nothing is provided and the Content-Type is application/json

What is the new behavior?

• @marblejs/testing - parse response body to undefined if nothing is provided and the Content-Type is application/json

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[codecov[bot]]

Codecov Report

Merging #389 (6517089) into master (cbdffda) will decrease coverage by 1.19%. The diff coverage is 92.98%.

@@            Coverage Diff             @@
##           master     #389      +/-   ##
==========================================
- Coverage   95.41%   94.22%   -1.20%     
==========================================
  Files         157      158       +1     
  Lines        2769     3097     +328     
  Branches      366      426      +60     
==========================================
+ Hits         2642     2918     +276     
- Misses        123      175      +52     
  Partials        4        4              

Impacted Files	Coverage Δ
packages/core/src/logger/logger.interface.ts	100.00% <ø> (ø)
packages/core/src/operators/index.ts	100.00% <ø> (ø)
packages/core/src/operators/use/use.operator.ts	50.00% <ø> (-50.00%)	:arrow_down:
packages/http/src/http.interface.ts	100.00% <ø> (ø)
packages/http/src/index.ts	0.00% <0.00%> (ø)
packages/http/src/router/http.router.combiner.ts	92.30% <ø> (ø)
packages/http/src/router/http.router.matcher.ts	100.00% <ø> (ø)
...ages/http/src/router/http.router.params.factory.ts	100.00% <ø> (ø)
packages/http/src/server/http.server.interface.ts	100.00% <ø> (ø)
packages/messaging/src/reply/reply.ts	100.00% <ø> (ø)
... and 122 more

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