logging: avoid '%!F(MISSING)' in logs (#6555)

When only passing a single string into a Printf-style function, any %-format
specs in it will be filled accordingly; but there will be no arguments to use.

So now, we'll avoid calling the `{Debug,Info,Warn,Error}f` functions when we
lack arguments.

Signed-off-by: Stephan Renatus <stephan@styra.com>