Fix NoMethodError when choosing a too big number from gem uni list

[amatsuda]

What was the end-user or developer problem that led to this PR?

Executing gem uni somegem and choosing a very big number that is not in the list (e.g. choosing 5 from the gem uni rails list where having only 2 rails version being installed) causes the following error:

ERROR:  While executing gem ... (NoMethodError)
    undefined method `>=' for nil

      elsif index >= 0 && index < list.size

This happens since abacb0c by @nobu where choose_from_list has been changed to return nil index when an out-of-bound number was chosen (it used to simply return the chosen index regardless of the list size), and so this error can be reproduced on either rubygems v3.4.18, v3.4.19, and 3.5.0.dev as of today.

What is your fix for the problem, implemented in this PR?

Just added a nil check before using the index value in Uninstaller#uninstall.

I skipped adding a reproduction test for this case because I couldn't find a good place to write such an integrated combination test of the ui and the command while unit test for each of them exist independently.

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• Write tests for features and bug fixes
• Write code to solve the problem
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