handle empty input for reval_p

[schlichtanders]

if nothing is actually given (like an empty string) this now behaves correctly in returning nothing

fixes #510

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.90%. Comparing base (1517803) to head (61bb39b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #523      +/-   ##
==========================================
- Coverage   80.40%   77.90%   -2.50%     
==========================================
  Files          26       26              
  Lines        1684     1684              
==========================================
- Hits         1354     1312      -42     
- Misses        330      372      +42     

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[palday]

@schlichtanders can you add a test of this behavior?

[schlichtanders]

@palday yes, please may you point me to the place in the code where you would like to have the test?

[palday]

@schlichtanders maybe just at the top of https://github.com/JuliaInterop/RCall.jl/blob/master/test/basic.jl ?