Generated ntp.conf has controlkey and requestkey lines with no values

[robert-cam]

Describe the Bug

When ntp::keys_controlkey and ntp::keys_requestkey are null the generated ntp.conf contains:
requestkey
controlkey

ntpd complains about these lines as syntax errors.

Expected Behavior

Don't output these lines if the variables are null.

Steps to Reproduce

Steps to reproduce the behavior:
Generate a config file with these variables left on the default null from common.yaml in the module.

Environment

• Version 10.0.0
• Platform CentOS 7.9

Additional Context

Testing for null with "unless $ntp::keys_controlkey.empty" in the template seems to fix this.