Added NullUniqueIdentifier struct to work with GUIDs that can be null

[eduolalo]

I'm working with the excecution of stored procedures, if the execution generates an transaction the answer will include a GUID, if it do not generates an transaction it does not mean there is an error, so I can or can't receive an unique identifier, thats why I think would be nice to include this data type in this proyect.

[codecov[bot]]

Codecov Report

Merging #635 (a8491d9) into master (045585d) will decrease coverage by 0.01%. The diff coverage is 77.27%.

@@            Coverage Diff             @@
##           master     #635      +/-   ##
==========================================
- Coverage   71.98%   71.97%   -0.02%     
==========================================
  Files          24       25       +1     
  Lines        5469     5491      +22     
==========================================
+ Hits         3937     3952      +15     
- Misses       1309     1314       +5     
- Partials      223      225       +2     

Impacted Files	Coverage Δ
nulluniqueidentifier.go	77.27% <77.27%> (ø)
tds.go	67.21% <0.00%> (-0.39%)	:arrow_down:
token.go	61.08% <0.00%> (+0.15%)	:arrow_up:

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