Add outer nullability to Union

[connortsui20]

Tracking issue: #8769

Splits the breaking Union dtype change out of #8791.

Following #8769, we are going to track both inner and outer / top-level nullability in union types.

Changes

• changes DType::Union to carry independent outer nullability
• keeps outer nullability separate from the nullability of each variant
• updates dtype coercion, serialization, display, scalar construction, and downstream matches
• distinguishes an outer null Union from a present Union whose selected child is null

This changes the serialized Serde representation of Union dtypes and will require the compatibility override to be reviewed explicitly.

[connortsui20]

Superseded by #8798. During the stack reorder, GitHub marked this PR merged into its former stacked base; it did not merge the nullability change into develop.