Make Unique constraint works with nans

[amontanez24]

Problem Description

As a user, it is confusing and inconvenient when my Unique constraint fails validation because of missing values.

Currently

• missing values are treated as unique values in the single column case
• missing values cause failure 100% of the time in the multi column case

We want to make the behavior for the Unique constraint consistent in both cases when handling missing values.

Expected behavior

We should treat missing values (np.nan, None, etc.) as a unique category/value. Consider the following cases:

Should PASS → YES (right now this fails)

Col1		Col2
A		NaN
NaN		B
C		D
NaN		NaN

Should this pass? → NO

Col1	
A		
NaN	
C	
NaN

Should this pass? – NO

Col1		Col2
A		NaN
A		NaN
NaN		B
C		D
NaN		NaN

Should this pass? – NO

Col1		Col2
A		NaN
NaN		B
C		D
NaN		NaN
NaN		NaN

Currently all these cases will fail validation, but the first one should pass