fix: hooks not being called

[rigor789]

closes NativeScript/nativescript-app-templates#227

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What is the current behavior?

The hooks service is initialized and caches the hooksDirectories, however in some cases this might be an empty array (ie. when creating a new project, and the service is initialized before the project files exist yet).

What is the new behavior?

We only cache when there's items in the hooksDirectories (since this means the projectDir was defined)

Fixes/Implements/Closes #[Issue Number].