Repro:

• Add Credentials
    - set Kind to "SSH Username with private key"
    - tick "enter directly"
    - paste a password-less private key without a trailing newline
• Attempt to use credentials (I used ssh-agent from a Jenkinsfile)
• Observe that ssh-add will prompt for a passphrase in the logs and the ssh-add has failed.

The relevant part of my logs looked like this:

```
[Pipeline] sshagent
[ssh-agent] Using credentials jenkins (Github SSH key)
[ssh-agent] Looking for ssh-agent implementation...
[ssh-agent] Exec ssh-agent (binary ssh-agent on a remote machine)
$ ssh-agent
SSH_AUTH_SOCK=/tmp/ssh-rEGjLSRTHULl/agent.3927
SSH_AGENT_PID=3929
[ssh-agent] started an agent
$ ssh-add /var/lib/jenkins/workspace/job@tmp/private_key_2980200938951827942.key
Enter passphrase for /var/lib/jenkins/workspace/job@tmp/private_key_2980200938951827942.key: [Pipeline] // sshagent
[Pipeline] }
[Pipeline] // stage
[Pipeline] }
[Pipeline] // withEnv
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
ERROR: Failed to run ssh-add
Finished: FAILURE

```

Adding the trailing newline to input in the web-ui resolves this issue. Adding multiple newlines didn't seem have any adverse effect so Jenkins should probably just add a newline when it writes the keyfile.