title: [robotframework]Why Create a empty element to a list can treat as True, but not applicable to a empty variable
body: <p>Create a empty element to a list, and return True value with &quot;Should Be True&quot; key word.  but not suiteable for create a empty variable.Why is that?
  code:</p>
  <pre><code>${list1}    Create List    ${EMPTY}
  ${list2}    Set Variable    ${EMPTY}
  Should Be True    ${list1}    Create empty list
  Should Be True    ${list2}    Set empty variable
  </code></pre>
  <p>Log:</p>
  <p>20211225 14:24:58.913 :  INFO : ${list1} = ['']</p>
  <p>20211225 14:24:58.916 : TRACE : Arguments: [ '' ]</p>
  <p>20211225 14:24:58.916 : TRACE : Return: ''</p>
  <p>20211225 14:24:58.917 :  INFO : ${list2} =</p>
  <p>20211225 14:24:58.918 : TRACE : Arguments: [ [''] | 'Create empty list' ]</p>
  <p>20211225 14:24:58.918 : TRACE : Return: None</p>
  <p>20211225 14:24:58.919 : TRACE : Arguments: [ '' | 'Set empty variable' ]</p>
  <p>20211225 14:24:58.920 :  FAIL : Evaluating expression '' failed: ValueError: Expression cannot be empty.</p>