For the ssh-user-private-key credential-wrapper, the "key-file-variable"
setting is mandatory, but the "username-variable" and
"passphrase-variable" settings are optional.
Prior to this commit, if a user omitted "username-variable" or
"passphrase-variable" settings, JJB would write empty
<usernameVariable/> or <passphraseVariable/> entries. When both were
empty, the credentials-binding plugin would crash with an error:
IllegalArgumentException("Cannot use the same key in both secretValues and publicValues")
To resolve this, do not write XML entries for these optional parameters
when the user does not specify them.
Add a minimal test case reproducer.
Change-Id: I94a4437eee6a8fdaf655c1a3bf69d46844b225bc
787164e09a