[A3] Fix pnpm: true

[myovchev]

Summary

Fix a problem when using pnpm option:

// app.js
require('apostrophe')({
  pnpm: true
});

that results in Apostrophe crash "can not invoke add of undefined".

What are the specific steps to test this change?

Adding the option above is not crashing the instance.

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[boutell]

LGTM