core: fixed missing field initializers

[tcoyvwac]

Your checklist for this pull request

• [x] I've read the guidelines for contributing to this repository
• [x] I made sure to follow the project's coding style
• [x] I've updated the documentation with the relevant information (if needed)

Detailed description

• When building the project from clean, compiler complains with:

    ./cutter/src/core/Cutter.cpp:845:45: warning: missing initializer for member ‘rz_str_stringify_opt_t::length’ [-Wmissing-field-initializers]
  845 |                 RzStrStringifyOpt opt = { 0 };
      |                                             ^                  

• This PR fixes said complaint by filling in the missing fields for the variable opt of datatype RzStrStringifyOpt.

Test plan (required)

• CI should pass and build with no problems.
  • Structurally, nothing has changed, only filled in missing data fields.

Closing issues

N/A

---