rpk: fix printing new lines

[r-vasquez]

If the message was empty, we would print new lines to stderr

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v24.2.x
• v24.1.x
• v23.3.x

Release Notes

• none

[twmb]

Merging quickly before tests -- improves stdout, and we want to catch the RC.

[vbotbuildovich]

/backport v24.2.x

[vbotbuildovich]

/backport v24.1.x

[vbotbuildovich]

non flaky failures in https://buildkite.com/redpanda/redpanda/builds/57484#0192e947-ba4d-4bd6-9fb6-8d29cf52f240:

"rptest.tests.control_character_flag_test.ControlCharacterNag.test_validate_nag_message.initial_version=.22.3.11"

[vbotbuildovich]

Retry command for Build#57484

please wait until all jobs are finished before running the slash command

/ci-repeat 1
tests/rptest/tests/control_character_flag_test.py::ControlCharacterNag.test_validate_nag_message@{"initial_version":[22,3,11]}

[vbotbuildovich]

ducktape was retried in https://buildkite.com/redpanda/redpanda/builds/57484#0192e938-334c-411e-b06d-4e3eba738742