Flow fails to handle Subtitle/Title == None (null) of Python plugins

[pc223]

When a Python plugins return a result with Subtitle/Title == None (Flow translate to null), Flow not show the result anymore.

Flow should treat None/null comes from plugins as empty string.

Because of this bug, isPrime plugin fails to show when input a prime number
https://github.com/lvonkacsoh/Flow.Launcher.Plugin.IsPrime

[pc223]

Seems this bug is fixed recently, can't reproduce in dev branch, will do further tests to confirm.

Seems #556 fix the issue already