smartcontract: allow to pass nil as parameter to (*Invoker).Call

[AnnaShaleva]

Problem

Error on passing nil as an argument to (*Invoker).Call

Solution

Merge.

[codecov]

Codecov Report

Merging #3067 (c926d53) into master (35ebdc1) will increase coverage by 0.04%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master    #3067      +/-   ##
==========================================
+ Coverage   84.64%   84.68%   +0.04%     
==========================================
  Files         329      329              
  Lines       43792    43827      +35     
==========================================
+ Hits        37069    37117      +48     
+ Misses       5220     5200      -20     
- Partials     1503     1510       +7     

Impacted Files	Coverage Δ
pkg/neorpc/types.go	81.25% <66.66%> (-11.25%)	⬇️
pkg/core/transaction/witness_scope.go	91.04% <100.00%> (+6.56%)	⬆️
pkg/smartcontract/parameter.go	92.94% <100.00%> (+0.04%)	⬆️

... and 7 files with indirect coverage changes

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