feat: Make variables without a default value mandatory

[Michel Feinstein]

Description

I want to force my users to provide some values, so empty strings are not acceptable, but currently, if I remove the default field, I am still able to provide empty values.

IMO if someone wants a value that can be empty, then they should provide it as default: "" or default:. If no default was provided, then the tool should enforce that users give it a value.

Requirements

• Mandatory variables.

[Michel Feinstein]

@felangel one more example on how this could be useful. I wanted to use this template:

defaults.project={{#sentryProjectName}}{{sentryProjectName}}{{/sentryProjectName}}{{^sentryProjectName}}{{packageName}}{{/sentryProjectName}}

But it doesn't work because sentryProjectName is always a string, so the string will always be defined and the {{^sentryProjectName}} option will never be triggered.

[Utpal Barman]

Yes, this will be super useful if we can include required