Allow Propshaft assets to use fallbacks

[ohrite]

We're longtime inline_svg users, and we've really enjoyed your work on this gem. Thank you!

When running Rails 7 using Propshaft, when an asset is not found, the asset path resolver returns nil, raising an exception, even whenfallback: is provided. This commit introduces a nil check around the asset path finder's return value, a lot like the one in StaticAssetFinder.

The higher-level FindsAssetPaths spec never calls down into the PropshaftAssetFinder itself, so this commit also introduces a test suite around the PropshaftAssetFinder.

[jamesmartin]

Thanks for your contribution, much appreciated! ✨