Enable clippy::default_constructed_unit_structs (#8778)

This PR enables the
[`clippy::default_constructed_unit_structs`](https://rust-lang.github.io/rust-clippy/master/index.html#/default_constructed_unit_structs)
rule and fixes the outstanding violations.

Release Notes:

- N/A

tooling/xtask/src/main.rs

@@ -85,7 +85,6 @@ fn run_clippy(args: ClippyArgs) -> Result<()> {
         "clippy::cast_abs_to_unsigned",
         "clippy::cmp_owned",
         "clippy::crate_in_macro_def",
-        "clippy::default_constructed_unit_structs",
         "clippy::derivable_impls",
         "clippy::derive_ord_xor_partial_ord",
         "clippy::eq_op",