fix: do not attempt substitution if the loader failed

[jacquesg]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/Nikaple/nest-typed-config/blob/main/CONTRIBUTING.md
• [x] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [ ] Related issue linked using fixes #number
• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

What is the current behavior?

When substitution of environment variables in enabled in the fileLoader, an exception is generated as the check for whether or not the config actually loaded is missing:

TypeError: Cannot read properties of null (reading 'config')                                                                                                                                              
    at Object.transform (/home/jacquesg/dev/projects/asterias/deianira/node_modules/nest-typed-config/lib/loader/file-loader.ts:120:41)

Issue Number: N/A

What is the new behavior?

The validity is checked, and the correct error is now generated:

/home/jacquesg/dev/projects/asterias/deianira/node_modules/nest-typed-config/lib/typed-config.module.ts:79
    return load();
           ^
Error: Failed to find configuration file.
    at /home/jacquesg/dev/projects/asterias/deianira/node_modules/nest-typed-config/lib/loader/file-loader.ts:135:5

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[github-actions[bot]]

:tada: This PR is included in version 2.9.2 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket:

[jacquesg]

Wow, that was fast, thank you :)