Handle empty package list during python installation

[corivera]

Fixes #25049

If there's an error when retrieving the list of python packages we return an empty array instead. In most places we iterate over this list, so being empty isn't an issue. In this spot we were using a find call though, which would return undefined when the array is empty. This change adds an undefined check before using the results of that find call.

[coveralls]

Pull Request Test Coverage Report for Build 7936952128

Details

• -3 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.001%) to 41.757%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
extensions/notebook/src/jupyter/jupyterServerInstallation.ts	0	3	0.0%

Totals
Change from base Build 7934141412:	-0.001%
Covered Lines:	30827
Relevant Lines:	69082

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💛 - Coveralls

[corivera]

I'll go ahead and merge this without waiting for sign offs, since people are out for the long weekend and it's a pretty simple change.