Check if a variable is empty in jinja

I want to check if a variable is empty array or has values in jinja.

I get a variable from python named projects_contrib which can be []. I tried to check this like this

   <h4> Case 2 </h4>

But here always is executes case 2 even where projects_contrib is [].

I also tried like this

   Liquid error: Unknown operator is 

But the same thing, only the case 2 is executed. I need help, what I'm doing wrong?