Do not throw warning on empty RzIL

[XVilka]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

To avoid warnings like that:

WARNING: rz_il_op_effect_stringify: assertion 'op & sb' failed (line 1019)

when there is instruction with empty RzIL.

Test plan

CI is green.