Don't panic on binaryResponses where the body wasn't set

[mstoykov]

This panic was still caught and didn't abort the whole k6 just the
current iteration.

[imiric]

Thanks!

[codecov-commenter]

Codecov Report

Merging #2041 (9b8cde2) into master (5fe5e5a) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2041      +/-   ##
==========================================
- Coverage   71.75%   71.67%   -0.09%     
==========================================
  Files         179      177       -2     
  Lines       14104    14099       -5     
==========================================
- Hits        10121    10106      -15     
- Misses       3345     3352       +7     
- Partials      638      641       +3     

Flag	Coverage Δ
ubuntu	71.67% <100.00%> (-0.03%)	⬇️
windows	?

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

Impacted Files	Coverage Δ
js/modules/k6/http/response.go	78.57% <100.00%> (ø)
js/common/initenv.go	66.66% <0.00%> (-33.34%)	⬇️
lib/testutils/minirunner/minirunner.go	76.08% <0.00%> (-4.35%)	⬇️
lib/netext/httpext/error_codes.go	94.52% <0.00%> (-2.74%)	⬇️
js/initcontext.go	90.42% <0.00%> (-2.13%)	⬇️
lib/executor/vu_handle.go	93.33% <0.00%> (-0.96%)	⬇️
js/runner.go	81.30% <0.00%> (-0.57%)	⬇️
cmd/ui_windows.go
lib/netext/httpext/error_codes_syscall_windows.go

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[mstoykov]

The test failures should be fixed by #2038