Updates the check for started in job header

What does this MR do?

During the refactor this function was updated wrongly from

return !this.job.started === false;

to

export const jobHasStarted = state => !(state.job.started === false);

Turns out we were expecting started to be false, but started is never false, which cause this to break. Started is sent as null when the job as not yet started, and as a string with a date when it has started.

This MR updates it to check if it's a string instead.

What are the relevant issue numbers?

#52614 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [n.a.] Conforms to the database guides