replace retval of ToDB with {}interface(nil)

[koron]

instead of []byte(nil).

current custom types can't be set to NULL.
because some drivers (like lib/pq) don't consider that []byte(nil) is NULL . when ToDB() return nil, it is []byte(nil).

This PR checks return values from ToDB(), and when it is []byte(nil)
translates those to {}interface(nil). it is treated as NULL

[codecov-io]

Codecov Report

Merging #1044 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1044      +/-   ##
==========================================
- Coverage   54.28%   54.24%   -0.05%     
==========================================
  Files          41       41              
  Lines        7551     7557       +6     
==========================================
  Hits         4099     4099              
- Misses       2936     2941       +5     
- Partials      516      517       +1

Impacted Files	Coverage Δ
session_convert.go	22.1% <0%> (-0.19%)	⬇️
statement.go	66.66% <0%> (-0.18%)	⬇️

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