fix: so empty string can not be used as default value

[davidsoderberg]

What change does this PR introduce?

so empty string can not be used as default value

Why was this change needed?

To be more logical with how we handle default values

[linear]

NV-1598 🐛 Bug Report: Template variables are given a default empty string value even when nothing is entered in the default value form

📜 Description

When creating a template, even when you do not set a default variable, the value is set to an empty string.

👟 Reproduction steps

Create a template and use a variable named testVar but do not enter a default value. Send testVar in your payload as null or undefined.

👍 Expected behavior

Novu should not overwrite with an empty string if there is no default value set.

👎 Actual Behavior with Screenshots

Note that when inspecting the "payload" in the activity feed Novu will have sent an empty string as a default value for testVar.

You can also check by getting the template from the API and seeing this in the variables:

            {
              "name": "testVar",
              "type": "String",
              "required": false,
              "defaultValue": "",
              "_id": "63d5ccb3a9b6cc61d4cdce39"
            },```

### 📃 Provide any additional context for the Bug.

_No response_

### 👀 Have you spent some time to check if this bug has been raised before?

- [X] I checked and didn't find similar issue

### 🏢 Have you read the Contributing Guidelines?

- [X] I have read the [Contributing Guidelines](https://github.com/novuhq/novu/blob/main/CONTRIBUTING.md)

### Are you willing to submit PR?

None

[scopsy]

Looks great, Is it possible to add a test for this functionality?

[p-fernandez]

🌟

[scopsy]

Shouldn't the message be created with the variable even if the default value is false? And we just shouldn't override the sent message with this information?

Because this variable is still was used in the template (if that's the usage of the variables array)

And only during trigger we should filter out to not override it with a string. Or maybe I'm missing something @davidsoderberg