UI: Allow a project variable to be set to an empty value

Originally opened at !6044 (closed) by @lnovy.

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What does this MR do?

This changes project variables UI forms to allow the variable's value to be set to an empty value.

Are there points in the code the reviewer needs to double check?

I've also robustified some other scenarios in the test spec besides adding a new one which tests the empty value.

Why was this MR needed?

Empty valued environment variables are perfectly valid and useful. Setting the value empty can also server as a poor-man's way to "unset" the variable if it was defined somewhere lower in the order of precedence as documented at [http://docs.gitlab.com/ce/ci/variables/README.html]

What are the relevant issue numbers?

None.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated not applicable
• API support added not applicable
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together