Seems to be fixed on Jenkins ver. 1.594.
I created a slave with the name "a b c" and a project for which I added the restriction to that slave. While typing the name of the slave I got the suggestion "a b c" without the quotes. Pressing ENTER, the name of the slave without the quotes was added to the text field. After saving the configuration of the job, the label appeared with quotes, which is correct. Furthermore, every functionality related to this label could be verified:
- the project was sent to the proper slave
- the project was tied to the assigned slave, when opening the page of the slave
So this is either fixed or it is something Jenkins handles automatically once the configuration page of the job is saved, since there was no need to add the quotes manually in the test.
Using quotes make sense due to more complex constructions: e.g:
user:... || role:... || "a b c"
Reproduced on 1.567:
Slave name: a b c
Error: Invalid boolean expression: unexpected token: b