code generator should not generate empty global-types

continuation of https://github.com/apollographql/apollo-tooling/issues/1179#issuecomment-616496523

**Intended outcome:**

`global-types.ts` cannot be empty as this breaks typescript compiler when `--isolatedModules` is set. This option can not be disabled for nextjs users

**Actual outcome:**

if there is nothing to put into `global-types.ts`, apollo generates an empty file. This throws an error on build `Type error: All files must be modules when the '--isolatedModules' flag is provided.`

**How to reproduce the issue:**

set `isolatedModules` and run a build

**Versions**

apollo 2.28.3

Reproduce the issue by enabling TypeScript's isolatedModules option and running a build that generates global-types.ts. Start by locating the code-generator entry point responsible for that file. Done means the generator no longer produces an empty global-types.ts and the build completes without the isolatedModules error.

Written by the indexing model from the issue text.