There is no way to hide an entire collection if all patterns need to be hidden

For example,

`src/patterns/foo` has two patterns, both of which should be hidden.

Adding a `collection.yaml` to this directory with the contents of `hidden: true` should make the `Foo` collection hidden from iterators, etc.

Currently, attempting to do this results in an error with the builder:

```
[10:45:18] Starting 'drizzle'...
[10:45:18] Starting 'css:drizzle'...
[10:45:18] Starting 'css:toolkit'...
[10:45:18] Starting 'js'...
[10:45:19] Starting 'assets'...
[10:45:21] Finished 'css:drizzle' after 2.6 s
[10:45:22] 'drizzle' errored after 3.8 s
[10:45:22] Error
at Error.DrizzleError (/Users/aileen/Desktop/jedec/node_modules/drizzle-builder/dist/utils/error.js:9:16)
at Function.DrizzleError.error (/Users/aileen/Desktop/jedec/node_modules/drizzle-builder/dist/utils/error.js:26:13)
at /Users/aileen/Desktop/jedec/node_modules/drizzle-builder/dist/parse/patterns.js:242:27
[10:45:24] Finished 'js' after 5.14 s
[10:45:24] gulp-imagemin: Minified 0 images
[10:45:24] Finished 'css:toolkit' after 5.41 s
[10:45:24] Finished 'assets' after 5.36 s
```

/CC @aileenjeffries @mrgerardorodriguez