fix(Debug): treat zero time as valid timer start

[gr8man]

Description
Timer::start() treated 0.0 as "no value" and silently replaced it with microtime(true). Changed to null coalescing (??) so only null falls back, preserving 0.0 as a valid timer start time.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[paulbalandan]

Please add a changelog entry.