fix modules.json and terraform-sources.json overriding each other

[ansgarm]

#1836 added support for parsing terraform-sources.json files present in stacks projects. As the modules.json and the terraform-sources.json files are mutually exclusive and thus not expected to be present at the same time, that PR re-used the existing InstalledModules for simplicity. However, it thereby accidentally introduced a race between the two jobs looking for and parsing these files. In the case of hashicorp/vscode-terraform#1915, the job parsing terraform-sources.json terminated second and overwrote InstalledModules with nil as no manifest file was found.

This PR adds a simple fix to only overwrite InstalledModules if it is not nil – this is just a short-term bandaid however, given it might lead to stale data. For a long-term fix, we should split InstalledModules into two fields instead.

Resolves hashicorp/vscode-terraform#1915

[github-actions]

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