Let's say you are using the vars plugin to create variables. Notice in this example that var2 contains null and var3 contains no value.

vars:
  var1: "Hello World"
  var2: null
  var3:

The debug module can be used to print the value of each variable.

- name: output the 'var1' variable
  debug:
    var: var1

- name: output the 'var2' variable
  debug:
    var: var2

- name: output the 'var3' variable
  debug:
    var: var3

Something like this should be returned. Notice that both var2 and var3 contain no value.

TASK [output the 'var1' variable]
ok: [server1.example.com] => {
    "var1": "Hello World"
}

TASK [output the 'var2' variable]
ok: [server1.example.com] => {
    "var2": ""
}

TASK [output the 'var3' variable]
ok: [server1.example.com] => {
    "var3": ""
}