Implement Default for AssumeNotNull

[dullbananas]

No description provided.

[Ten0]

Just not sure to what extent this is backwards compatible - can't users implement that in their own crate if they own T?

[weiznich]

That should be backward compatible. At least the following code does not compile in a downstream crate with an error message related to the fact that you cannot implement a foreign trait for a for foreign type.

struct Foo;

impl Default for diesel::dsl::AssumeNotNull<Foo> {
    fn default() -> Self {
        todo!()
    }
}

This only works if the trait itself has a type parameter or if the type is marked at fundamental. (Beside of that downstream users wouldn't be able to provide a meaningful implementation of Default for that type, as it exposes no public fields and no constructor.)