PR: Allow null channels in file to not crash setup.

[Lnaden]

There is an edge case where having channels in an environment file
but an empty list, and channels missing from the GHA options/keys
returns an unhelpful error message:

Error: Cannot read property 'join' of null

I've setup an example here which runs through all the permutations of
channels keys being set, missing, and nulled:

https://github.com/Lnaden/ghatesting/runs/1538195898?check_suite_focus=true

This PR attempts to fix that by relaxing the strict equals in setup.ts
from

if (condaConfig["channels"] === "" && channels !== undefined) {

to

if (condaConfig["channels"] === "" && channels != null) {

which should allow channels != null to catch both null and
undefined for channels. I also attempted to add an example to the
GitHub Actions tests (Example 9) to test this edge case specifically.

A few comments to this PR:

• I'm not familiar with Typescript and building GHAs, so this might not be correct and I'm happy to make changes and take feedback.
• If you use an env file with an empty channels list with the conda client to create an environment, it works fine and just defaults to whatever the conda is configured to. In my example above, setting no channels in either the file or the GHA keys works fine and pulls from the default channel.
• This behavior admittedly is ambiguous, so if you would rather this edge case throw an error, I would like to request a better error message.
• The package.json and package-lock.json files may not need to be merged with this? I followed the instructions on the CONTRIBUTING.md file and these wound up getting changed. Is that expected (e.g. 11k lines on package-lock)?

[goanpeca]

@Lnaden thanks a lot for working on this fix :)

I updated your PR and rebased with latest master to solve the conflicts.

[goanpeca]

Any thoughts @bollwyvl :) ?

[bollwyvl]

I'm pretty sure this is now covered by:

https://github.com/conda-incubator/setup-miniconda/blob/master/src/conda.ts#L97

  let channels = inputs.condaConfig.channels /* plus some string stuff */

  if (!channels.length && options.envSpec?.yaml?.channels?.length) {
    channels = options.envSpec.yaml.channels;
  }

[goanpeca]

I will update to just use the sample yaml.

[Lnaden]

Awesome, glad this got resolved regardless of how.