[Delta] When checks schema for writing, Delta enforces not null on a Nested Field only when its parent is not null

[harperjiang]

Which Delta project/connector is this regarding?

• Spark
• Standalone
• Flink
• Kernel
• Other (fill in here)

Description

This PR corrects Delta's handling of nullable checks for nested fields when writing data.

For example, consider the schema:

STRUCT<foo: STRUCT<bar: INT NOT NULL, bar2: INT>>

Previously, Delta rejected NULL values for foo, even when it was marked as nullable. This was because a NULL foo would also make foo.bar NULL.

With this change, Delta now enforces foo.bar as non-null only when foo itself is non-null. As a result:

• foo = NULL is now accepted.
• foo = { bar = NULL, bar2 = 1 } remains rejected.

How was this patch tested?

UT

Does this PR introduce any user-facing changes?

No