Allow null or empty values in a variable using Ansible

I have this piece of code that checks if the variable "owner" matches with the following regex and accepts that it is undefined, that is, the playbook can work without that variable being passed.

varexample is undefined or varexample is match('^[a-zA-Z]+$')

What I would like to do is that this variable accepts empty or null values. What I tried is something like this

varexample is null or varexample is match('^[a-zA-Z]+$')

But I got this error:

The conditional check 'varexample is null or varexample is match('[a-zA-Z]+')' failed. The error was: template error while templating string: no test named 'null'. String: {% if varexample is null or varexample is match('[a-zA-Z]+') %} True {% else %} False {% endif %}

Can someone give me a hint or some help?

Q: "variable accepts null values"

A: It's a bug. Ansible shouldn't match null to '^[a-zA-Z]+$'

    - set_fact:
        varexample:
    - debug:
        var: varexample
    - debug:
        msg: "undefined: {{ varexample is undefined }}"
    - debug:
        msg: "match: {{ varexample is match('^[a-zA-Z]+$') }}"

gives

  varexample: null
  msg: 'undefined: False'
  msg: 'match: True'

As a result of this bug, your condition should be working as expected

  varexample is undefined or varexample is match('^[a-zA-Z]+$')

To be on the save side, for the case the bug will be fixed, you can additionally test None, e.g.

    - debug:
        msg: "Test passed. varexample: {{ item.varexample }}"
      when: item.varexample is undefined or
            item.varexample == None or
            item.varexample is match('^[a-zA-Z]+$')
      loop:
        - varexample: ABC
        - varexample: 123
        - varexample:

gives

ok: [localhost] => (item={'varexample': 'ABC'}) => 
  msg: 'Test passed. varexample: ABC'
skipping: [localhost] => (item={'varexample': 123}) 
ok: [localhost] => (item={'varexample': None}) => 
  msg: 'Test passed. varexample: '

Details

    - debug:
        msg: |
          Undefined: {{ item.varexample is undefined }}
          Is None: {{ item.varexample == None }}
          Match a-zA-Z: {{ item.varexample is match('^[a-zA-Z]+$') }}
      loop:
        - varexample: ABC
        - varexample: 123
        - varexample:

ok: [localhost] => (item={'varexample': 'ABC'}) => 
  msg: |-
    Undefined: False
    Is None: False
    Match a-zA-Z: True
ok: [localhost] => (item={'varexample': 123}) => 
  msg: |-
    Undefined: False
    Is None: False
    Match a-zA-Z: False
ok: [localhost] => (item={'varexample': None}) => 
  msg: |-
    Undefined: False
    Is None: True
    Match a-zA-Z: True