Setting for the number of threads does not accept 0

[adityasharad]

Describe the bug
@yoff reports that the settings UI for the number of threads when running queries or tests does not accept 0 as input.

Expected behavior
0 is passed to the corresponding CLI commands, where it is interpreted as 'use one thread per core on the machine'.
A positive integer continues to be used as-is, and a blank setting defaults to 1.

Additional context
CLI docs for --threads argument in execute query-server and test run.

[yoff]

Thanks 🎉