contacting professors when looking for a job even if there are no advertised vacancies in their group
Your question is rather vague -- what does "is it a good idea" mean? If you're asking if you can write to professors? Sure; there's nothing to lose, and there may be people looking for lab technicians. (I've been in this situation of looking for someone to hire in the past.) Go for it! If you're asking if it's likely that you'll get a positive response: no, for all the reasons you state. 

As for how long/detailed such an email should be: you should succinctly (in a paragraph or two) state your skills and goals and possibly point out abilities that are particularly useful for the group you're writing to. Attach a clear CV.

I should point out that the statement "I haven't published any papers during my PhD" is rather worrying, and it may be a good idea to explain this when writing. Good luck!