Fix C-Bindings not properly handling the event of a null decoder

[SuperManifolds]

Description

C-bindings for decoding did not properly handle the event of a null decoder, as in they did not actually early return from the functions, continuing into a possible null reference exception. This fixes this issue by adding early returns.

Fixes Issue FRPC-8

Type of change

Please update the title of this PR to reflect the type of change. (Delete the ones that aren't required).

• Bug fix (non-breaking change which fixes an issue) [title: 'fix:']
• New feature (non-breaking change which adds functionality) [title: 'feat:']
• Refactor (non-breaking change which does not add functionality) [title: 'refactor:']
• Breaking change (fix, feature, or refactor that would change existing functionality or interfaces) [title: 'BREAKING CHANGE:']
• This change requires a documentation update

Final Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings
• I have signed-off my commits with git commit -s (see the developer's certificate of origin)

[ShivanshVij]

LGTM!