Eliminate this = default shortcut when height * width = 0

[marcin-krystianc]

Closes #1101

Previously, when creating 2D memory structures (Memory2D<T>, ReadOnlyMemory2D<T>, Span2D<T>, ReadOnlySpan2D<T>) with zero height or width, some constructors would take a shortcut by setting this = default. This approach had issues because it didn't properly preserve non-zero dimensions (e.g., a 4×0 or 0×7 structure would become 0×0). Additionally, the OverflowHelper.ComputeInt32Area method could return non-zero values when height=0 due to an invalid formula.

Solution

• Removed the this = default shortcuts from all 2D memory structure constructors

• Fixed calculation in OverflowHelper.ComputeInt32Area:

• Added comprehensive tests to verify empty structures maintain correct dimensions

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes
• Code follows all style conventions

Other information