fix: ValueWrapper.cast support __EMPTY__ type

[wey-gu]

it now calls as_empty which doesn't exist.

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

n/a

Description:

Previously it could not handle empty properly.

How do you solve it?

• Removed the as_empty call from the attribute map
• return None when it's empty

[codecov-commenter]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.03 ⚠️

Comparison is base (6e5742f) 77.62% compared to head (df081f3) 77.60%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #269      +/-   ##
==========================================
- Coverage   77.62%   77.60%   -0.03%     
==========================================
  Files          18       18              
  Lines        2409     2411       +2     
==========================================
+ Hits         1870     1871       +1     
- Misses        539      540       +1     

Impacted Files	Coverage Δ
nebula3/data/DataObject.py	76.35% <50.00%> (-0.07%)	⬇️

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