Environment Variables Will Not Display When An Empty Value Is Set

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Bug report

Describe the bug

When an empty value is set in the environment variables section of an app, the list will no longer be rendered.

To Reproduce

Create an app in Ledokku
Navigate to Environment Variables
Set any key, and an empty value
Save it and refresh page

Expected behavior

The environemnt variable page should still list all enviornment variables and a empty value for the set key

Screenshots

If applicable, add screenshots to help explain your problem.

System information

• Version of dokku: [e.g. 0.21.4]
• Version of ledokku: 0.7.0

Additional context

This can be assigned to me for fixing.