BUG: Fix possible undefined behavior in Transform::ApplyToImageMetadata

[blowekamp]

Backport of #6583 to release-5.4.

itk::Transform::ApplyToImageMetadata() calls this->GetInverseTransform()
and dereferences the result without checking for null. For a non-invertible
transform (e.g. a singular AffineTransform), GetInverseTransform()
returns a null Pointer, so this is an unchecked null dereference leading
to undefined behavior / a crash instead of a well-defined exception.

This adds a null check that throws an itk::ExceptionObject when the
transform is not invertible, matching the documented behavior of the
method.

Fixes B25 in #6575.

This was found while backporting SimpleITK PR #2721
(Transform::ApplyToImageMetadata) to SimpleITK's release branch, which
pins ITK to v5.4.7. SimpleITK's TransformTest.ApplyToImageMetadata unit
test exercises this exact case (EXPECT_ANY_THROW on a singular
transform), and segfaults without this fix.

Cherry-picked cleanly from 4365356 with a
one-line conflict resolution (an unrelated const qualifier added on
release-5.4's version of the same line).

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Adds null check to image metadata transform code.

The PR appears safe to merge, though the exception path still lacks a regression test.

Findings

1.  Missing regression test ▶

Summary

This backport checks the inverse transform before applying it to image metadata, replacing a null dereference with an ITK exception for non-invertible transforms.

• The existing ITK tests do not exercise the new exception path.

_{Reviews (2) · Last reviewed commit: "BUG: Fix possible undefined behavior in ..."}

[greptile-apps]

Missing regression test The new exception path is not covered by a test. The existing ApplyToImageMetadata test uses an invertible transform, and the singular-transform test checks only inverse retrieval. A test that calls ApplyToImageMetadata with a singular transform and expects an itk::ExceptionObject would catch a future return of the crash this change fixes.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[blowekamp]

Closing/reopening to retrigger ITK.macOS.Python, which failed with an unrelated compiler crash (clang segfault compiling Modules/ThirdParty/KWSys/src/KWSys/Status.cxx, unrelated to this change) rather than a real build/test failure.

[dzenanz]

You can trigger rerun of azure pipeline by posting a comment /azp run ITK.macOS.Python.