apollo-client is not compatible with strictNullChecks

[PowerKiKi]

Intended outcome:

In preparation to create a PR to solve #5662, I would like to compile apollo-client with strictNullChecks enabled in tsconfig.json.

Actual outcome:

npm run build will fail with the message Found 91 errors.. All errors are of course related to strictNullChecks.

strictNullChecks was originally introduced in 2017 via #1221, but it seems it has been unintentionally removed in 2018 via 53230f4#diff-3daa69c0bea047c36f52994f7e96f2b2L8.

Unfortunately since that times several errors were introduced that makes the project not compatible with strictNullChecks anymore.

I am willing to attempt to create a PR to restore strictNullChecks, but first I need to be sure that it would get merged, since it seems to be a rather significant work.

Is there any interest to merge something like that ?

How to reproduce the issue:

Add "strictNullChecks": true, in tsconfig.json, and then run npm run build.

Versions

On latest master: f3c149c.

  System:
    OS: Linux 4.15 Ubuntu 18.04.4 LTS (Bionic Beaver)
  Binaries:
    Node: 12.16.1 - /usr/bin/node
    Yarn: 1.22.4 - /usr/bin/yarn
    npm: 6.13.4 - /usr/bin/npm
  Browsers:
    Chrome: 80.0.3987.149
    Firefox: 74.0

[PowerKiKi]

Fixed in #6080