defmt::println messages have a leading space when timestamps are disabled

[japaric]

Steps to reproduce:

• start from app-template (main branch)
• configure it to use git version of defmt crates
• remove the defmt::timestamp! item from src/lib.rs
• modify src/bin/hello.rs to use println!

defmt::println!("Hello, world!");

This outputs:

 Hello, world!
└─ hello::__cortex_m_rt_main @ src/bin/hello.rs:8

Ideally, it should output: (the H is exactly on top of the "corner pipe")

Hello, world!
└─ hello::__cortex_m_rt_main @ src/bin/hello.rs:8

to match the other logging macros, e.g. the levels example produces:

ERROR error
└─ levels::__cortex_m_rt_main @ src/bin/levels.rs:12

cc @justahero

[jonas-schievink]

Problem lies here:

defmt/decoder/src/log.rs

Line 59 in 4e59403

writeln!(&mut sink, "{} {}", timestamp, display).ok();

The fix is to make timestamp an Option<String>in that function and only add the space in that branch when it's Some.