[BUG] Change default value for header parameter

[gcatanese]

Fix #15989

When a property has no default value the string "null" is used as default.

This PR changes the getPropertyDefaultValue for strings from "null" to "" (empty string)

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[wing328]

this fix is mainly for the postman collection generator, right?

if that's the case, I don't think we should update default codegen.

What about overriding the method in postman collection generator instead?

[gcatanese]

Yes, that's an option but this applies to all generators IMO. When a parameter has no default the default codegen provides "null" string as default, for each type. Event for strings the default "null" is not a great option.
Looking at the comments in the code it looks to me they are supposed to be addressed at some point.

My suggestion would be to return null instead of the "null" string for each type. That's better as it respects the OpenAPI specs (if it doesn't specify a default value why would we generate one?) and let the generators decide what to do.
Let me know what you think, I can refactor in this way (return null) or go for the custom Postman fix.

[wing328]

Looking at the comments in the code it looks to me they are supposed to be addressed at some point.

The comment simply said the code block is to illustrate how to handle default value for different type so that generators can override this method easily.

When a parameter has no default the default codegen provides "null" string as default, for each type

The default codegen was created primarily for java client/server generators so it won't meet the requirement of generators in different languages. That's why generators should override toDefaultValue instead.

This PR changes the getPropertyDefaultValue for strings from "null" to "" (empty string)

I don't think the header parameter should default to "" (empty string) in most generators as the header with "" as the default value will now become

x-just-a-header: 

as opposed to simply omitting the header in the HTTP request.

Updating something in default codegen requires lots of tests and reviews and in this case I would suggest simply overriding toDefaultValue to meet your requirement in the postman collection generator.

[wing328]

FYI. I've fled #16003 to clean up the code a bit to avoid confusions.

[gcatanese]

@wing328 thanks for the clarifications.

The issue is that the default value "null" does not omit the header, and instead sends

x-just-a-header: "null"

I thought that would be an issue for most generators.

Anyway, I followed your suggestion and dealt with this within the Postman generator, please see the changes.