Return empty map for empty doc (instead of error)

[cooperka]

Graphql can't parse empty documents (e.g. from js files with no gql tags); return an empty OutputMap in these cases instead of failing. Example:

# File with no `gql` tags:
yarn run persistgraphql --js --extension=js src/components/App.js

leads to

Unable to process input path src/components/App.js. Error message:
Syntax Error GraphQL request (1:1) Unexpected <EOF>

1:
   ^

This makes it impossible for me to recursively analyze a directory (e.g. src/) because there are undoubtedly going to be some .js files in that directory without any gql tags.

Note: This is a similar error to #48 and #20 but seemingly a different cause. This was also merged recently but seemingly didn't fix the issue in all cases: #48.