[fix](be) Support null-only reads for not-null columns

[englefly]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:
Problem Summary: Queries can request a .NULL access path for a NOT NULL base column after outer-join nullability propagation when only the post-join null flag is needed. The scalar FileColumnIterator NULL_MAP_ONLY path assumed that the destination column was always nullable and failed in lazy materialization with Check failed: dst->is_nullable(). For NOT NULL storage columns, the null map is known to be all false, so materialize default non-null placeholders for non-nullable destinations while preserving the existing nullable null-map path.

Release note

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Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[englefly]

run buildall

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