fix(utils): avoid to use default value for null in json-templates

[mytharcher]

Description (Bug 描述)

Null value becomes undefined and cause comparison failure.

Steps to reproduce (复现步骤)

1. Create a condition node in workflow.
2. Assign left operand as a nullable variable.
3. Assign right operand as null constant.
4. Use "Equal" method.
5. Submit data contains the null variable (field).
6. Condition result is false and workflow fails.

Expected behavior (预期行为)

Condition result should be true.

Actual behavior (实际行为)

Condition result is false.

Related issues (相关 issue)

None.

Reason (原因)

The logic of default assign for null value in json-templates is not correct.

Solution (解决方案)

Use different way to deal with undefined and null.

[height]

This pull request has been linked to 1 task:

• T-813 条件节点比较 null 值相等结果错误 (unlink task)

💡Tip: Add "Close T-813" to the pull request title or description, to a commit message, or in a comment to mark this task as "Done" when the pull request is merged.