Fix stitchSchemas for when empty array of typeDefs are passed

[rufman]

If an empty array is passed, the GraphQL parser fails, because the
source is empty. Switch the typeDef check to check for length as well,
since an empty array is truthy.

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

[ardatan]

Thank you @rufman ! That was an issue on GraphQL Mesh.
ardatan/graphql-mesh#515

[theguild-bot]

The latest changes of this PR are available as alpha in npm: 6.0.6-alpha-e08738d.0

Quickly update your package.json by running:

npx match-version @graphql-tools 6.0.6-alpha-e08738d.0

[ardatan]

Available in v6.0.6 !