fix(misconf): fix infer type for null value

[nikpivkin]

Description

An example of a CloudFormation template for which parsing fails:

AWSTemplateFormatVersion: 2010-09-09
Parameters:
  Email:
    Type: String
Conditions:
  SubscribeEmail: !Not [!Equals [ !Ref Email, ""]]

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).