fix: ns update version null

[janoshrubos]

fixes #5440

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• Tests for the changes are included.

What is the current behavior?

We no longer specify the runtime versions in the templates, so when CLI tries to update the runtimes it fails.

What is the new behavior?

The CLI no longer reads the non-existent nativescript key in the package.json, instead the runtime defaults to the latest version.