Hypothetically, given developer tooling, which would you prefer to make sure your code is as correct and robust as possible?

1. Language with strong type features.

2. Testing packages that automatically include fuzz tests and null tests to ensure correct behavior?

Granted most of my experience in production level software involves backend services, which essentially means json transformations 95% of the time, Ive rarely ever seen an error that results in the service saying everything is ok but returning a wrong result. For example in the case of null fields, something downstream fails and errors out - this is easily caught by testing.