Add property value nullable constraint

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We are allowed to add schema constraint that a property key must belong to the vertex that has specific vertex labels, can we add a new constraint that a property key must be present(cannot be null)? This will help to increase the data robust without adding extra detect in some cases(like User vertex must have a username). I think the rule of property key that is not nullable should be:
- If a property key belong to specific vertex labels or edge labels, all vertices and edes of those labels must be provided the property.
- If a property key doesn't belong to any specific vertex/edge labels, all vertices/edges in the graph must be provided the property.

No files, tests, or entry points are named. Start by reviewing the existing schema-constraint implementation and its tests to understand how label-specific and graph-wide property rules are represented. Done means enforcing non-null properties for the specified vertex or edge labels, or across the graph when no labels are specified.

Written by the indexing model from the issue text.