When trying to export an endpoint that has no mutations, graphql-tools crashes while trying to parse and create the schema.
For example a DB using postgraphile that is exporting not tables, only a view.

To reproduce the bug:

1. Create a postgres db with some tables.
2. Create a postgres view.
3. Omit all the tables using "@omit read,update,create,delete,all,many" (https://www.graphile.org/postgraphile/smart-tags/#the--character)
4. Run graphql-tools

I am using graphql-mesh and it crashes when graphql-tools is trying to create the schema:
`

start
graphql-mesh serve

🕸️ => ⠋ Generating Mesh schema...GraphQL Mesh Configuration is not valid:
data/sources/0/handler/postgraphile must NOT have additional properties
🕸️ => ✖ Unable to start GraphQL Mesh: Cannot convert undefined or null to object
`

The expected behaviour should be ignoring the case of having no mutations and parsing only the queries.

Workaround:
Creating a simple table with no data (so you won't be afraid to export it). Don't omit the operations on this table.