Ensure Gitlab::Popen raises error for incorrect arguments

What does this MR do and why?

A HackerOne report which arose due to developers using .join() on an array of system command, and then using it for Gitlab::Popen as [command], essentially bypassing the point of Gitlab::Popen. I am adding a check to ensure that there are no whitespaces in the array, if there is only one item in the array. The vulnerable code snippet was like so:

command = %W("kubectl #{user_input}")
Gitlab::Popen.popen([command.join(' ')]) # this is essentially popen(["kubectl #{user_input}"])

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