Fix creating optional file

[KyoheiG3]

Fixes #546.

[yonaskolb]

Hi @KyoheiG3. I'm not sure what this fixes?

[yonaskolb]

This isn't checking if a directory exists, so it would always return nothing

[ra1028]

Hi @yonaskolb

This pull request is my colleague's.
We want to fix to reference auto-generated files that hasn't been generated yet when runs xcodegen.
For example, case of generate localized strings file using SwiftGen in build phases.
Requires to generate a Xcode project before runs SwiftGen in the pre-build phase.
So we expect like following:

How should we fix?

[AlexisQapa]

I think there is a pre gen hook or at least it was mentioned in another issue. Right now I deal with this by either having blank files (we don't use swiftgen for storyboard so files are known in advance) or running swiftgen once before Xcode gen.

[yonaskolb]

Comments left in #546
@AlexisQapa suggestions should resolve this issue.
Closing this for now