Simplify is_some() && …unwrap() to is_some_and in unit_arg

[shulaoda]

Summary

• is_expr_default_nested in clippy_lints/src/unit_types/unit_arg.rs was
  written as block.expr.is_some() && is_expr_default_nested(cx, block.expr.unwrap()),
  which is exactly the pattern that clippy's own unnecessary_unwrap lint flags.
• Replace it with block.expr.is_some_and(|e| is_expr_default_nested(cx, e)).

changelog: none

[rustbot]

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[llogiq]

Thanks.