[BUG] [C] _convertToJSON returns NULL if field is numeric and has value 0

Description

The C generator accidentally treats a numeric field like a pointer
in _convertToJSON and returns NULL whenever that field has value 0.
This applies to all numeric fields (dobue, int) with value 0
when _convertToJSON is called.

openapi-generator version

7.15.0

Suggest a fix

In modules/openapi-generator/src/main/resources/C-libcurl/model-body.mustache
at line 425 we have

    {{^isEnum}}
    if (!{{{classname}}}->{{{name}}}) {
        goto fail;
    }
    {{/isEnum}}

This is likely wrong and should be something like:

    {{^isEnum}}
    {{^isNumeric}}
    if (!{{{classname}}}->{{{name}}}) {
        goto fail;
    }
    {{/isNumeric}}
    {{/isEnum}}

simply because failing when a numeric value is 0 is plain wrong.

Start in modules/openapi-generator/src/main/resources/C-libcurl/model-body.mustache at line 425 and inspect how numeric fields are handled in _convertToJSON. Reproduce the reported behavior with a numeric field whose value is 0, then verify that generated C code no longer returns NULL for that valid value.