Logging: Don't complain about serverinfo if no --serverinfo is given

[hoffie]

Describe the bug

Jamulus -s -n logs Ignoring invalid serverinfo, please verify the parameter syntax..

To Reproduce

Expected behavior
No error should be logged if no --serverinfo is given.

Screenshots

Operating system
Linux

Version of Jamulus
master

Additional context
Issue got introduced in 5b2c2de.

[pljones]

@hoffie how is progress on this?