Yep, you make a good point regarding testing. While static typing will pick up problems, it's not going to save you from the infamous Null Pointer Exception!
Type inference with static types definitely feels like you get the best of both worlds. While not exactly in the functional realm, it's one of the reasons I've been looking at Kotlin for future projects.
That's another good thing about F# and Elm. Null is not an allowable value***. Instead you use Option/Maybe when a value may not exist. That also explicitly forces you to deal with the non-existent case. You can't forget, because it won't compile.
*** F# is on .NET. Most .NET libraries are imperative. Especially at the boundary of applications (deserialization, loading from DB) you still have to deal with nulls. But your core code can be free of them.
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