mini-browser: fix host pattern logic

[paul-marechal]

When we set environment variables in our launch.json file, we use a
notation like "VAR": "${env:VAR}". VS Code will replace the template in
the value string with the content of the actual env var value, and if
not set it will expand to "". This caused issues with the logic that uses
this value since we expected to get undefined instead of an empty
string.

Closes #9052

How to test

Run the Launch Electron Backend debug configuration, it should work with this patch.

Without it should fail with "Failed to get cookie domain".

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

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[vince-fugnitto]

I confirmed that #9052 is fixed with the following changes, I am now able to 'launch electron backend' from the launch configuration.

[kittaakos]

Thanks for the fix.

What about this location? Is this looking good?

theia/packages/mini-browser/src/node/mini-browser-endpoint.ts

Line 202 in be686ee

const pattern = process.env[MiniBrowserEndpointNS.HOST_PATTERN_ENV] ?? MiniBrowserEndpointNS.HOST_PATTERN_DEFAULT;

[paul-marechal]

@kittaakos we should change it too indeed. After looking into it, what doesn't help is that we set env variables in our launch.json that expand to empty string when vars are not set, which doesn't really sound useful to me. In most cases the env var should either be set or unset. I'll make another PR to address both points.

[kittaakos]

I'll make another PR to address both points.

Thank you for looking into it and for the future fix.

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Some context:
It did not cause any issues, I am a just few commits behind the master HEAD, and have to manually apply your fix in the source: I accidentally found this occurrence. So no hurry with the fix.