UNIQUE INDEX on nullable properties

[qomhmd]

I think a unique index should not put a restriction on null values for nullable properties.

CREATE CLASS Concept EXTENDS V;
CREATE PROPERTY Concept.name STRING;
CREATE PROPERTY Concept.loinc STRING;
CREATE PROPERTY Concept.sctid STRING;
CREATE PROPERTY Concept.alias EMBEDDEDLIST STRING;
CREATE INDEX Concept.name UNIQUE;
CREATE INDEX Concept.loinc UNIQUE;
CREATE INDEX Concept.sctid UNIQUE;

CREATE VERTEX Concept set name = 'symptom', loinc = '75325-1'; 
CREATE VERTEX Concept set name = 'condition', loinc = '75323-6'; -- is not executed because of the previous record with sctid = null
CREATE VERTEX Concept set name = 'complaint', loinc = '75322-8'; -- is not executed because of the previous record with sctid = null
CREATE VERTEX Concept set name = 'age group', loinc = '46251-5'; -- is not executed because of the previous record with sctid = null