Fix empty index path crash

[bryankeller]

Details

This fixes #76 - a bug where MagazineLayout was assuming that it would only receive "valid" index paths when requesting layout attributes. The crash would occur if a developer passed IndexPath() into layoutAttributesForItemAt. Flow layout avoids this crash by defaulting to IndexPath(item: 0, section: 0).

Part of me feels like this is just flow layout working around some weird behavior / maybe trying to mirror a workaround table view had from the beginning. The documentation for IndexPath looks like this:

extension IndexPath {

    /// Initialize for use with `UITableView` or `UICollectionView`.
    public init(row: Int, section: Int)

    /// Initialize for use with `UITableView` or `UICollectionView`.
    public init(item: Int, section: Int)

    /// The section of this index path, when used with `UITableView`.
    ///
    /// - precondition: The index path must have exactly two elements.
    public var section: Int

    /// The row of this index path, when used with `UITableView`.
    ///
    /// - precondition: The index path must have exactly two elements.
    public var row: Int

    /// The item of this index path, when used with `UICollectionView`.
    ///
    /// - precondition: The index path must have exactly two elements.
    public var item: Int
}

It specifically calls out that you should use the init(item: Int, section: Int) initializer when using UICollectionView. To me, this makes it seem like using IndexPath() is wrong / a programmer error.

====

Why does collection view / table view even use index paths? Seems like the wrong abstraction to me. IndexPaths allow indexing into arbitrarily deep tree structures, which seems completely overboard for a UI component that only has 2 layers (section and item).

Fun reminder than initializing IndexPaths is 35x slower than initializing a simple ElementLocation struct. See https://github.com/airbnb/MagazineLayout/blob/master/MagazineLayout/LayoutCore/Types/ElementLocation.swift#L23

Related Issue

#76

Motivation and Context

It matches UICollectionViewFlowLayout behavior, so I guess we should implement this workaround.

How Has This Been Tested

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.