smartcontract: fix bugs in paramter marshal/unmarshal

[fyrchik]

1. adjust values to types properly
2. allow to marshal nil byte slices

[codecov]

Codecov Report

Merging #719 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #719      +/-   ##
==========================================
- Coverage   64.03%   64.03%   -0.01%     
==========================================
  Files         132      132              
  Lines       11696    11695       -1     
==========================================
- Hits         7490     7489       -1     
  Misses       3913     3913              
  Partials      293      293

Impacted Files	Coverage Δ
pkg/smartcontract/param_context.go	75.2% <100%> (+0.3%)	⬆️
pkg/smartcontract/param_type.go	91.39% <100%> (-0.23%)	⬇️

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