Fix DiscardLocal client reset failure with unset callbacks

[ironage]

Fixes #5223
I forgot to check a function before calling it. I think SDKs other than cocoa have these always set because they wrap them up in a SDK layer callback so are likely not affected.

☑️ ToDos

• 📝 Changelog update
• 🚦 Tests (or not relevant)

[ejm01]

Code factor says "Complex Method"?
And a seemingly unrelated test has Command failed: couldn't read report file '/data/mci/bdfc013337a55c1faabd97af1ec7bd4f/realm-core/object-store-tests_results.json': unexpected end of JSON input?

Otherwise everything LGTM.

[ironage]

I fixed the complexity issue by refactoring a bit. I reluctantly agree that CodeFactor was helpful here.