style(3p/nix): Add braces around single-line conditionals

These were not caught by the previous clang-tidy invocation, but were
instead sorted out using amber[0] as such:

    ambr --regex 'if (\(.+\))\s([a-z].*;)' 'if $1 { $2 }'

[0]: https://github.com/dalance/amber

third_party/nix/src/libutil/ref.hh

@@ -17,11 +17,15 @@ class ref {
   ref<T>(const ref<T>& r) : p(r.p) {}
 
   explicit ref<T>(const std::shared_ptr<T>& p) : p(p) {
-    if (!p) throw std::invalid_argument("null pointer cast to ref");
+    if (!p) {
+      throw std::invalid_argument("null pointer cast to ref");
+    }
   }
 
   explicit ref<T>(T* p) : p(p) {
-    if (!p) throw std::invalid_argument("null pointer cast to ref");
+    if (!p) {
+      throw std::invalid_argument("null pointer cast to ref");
+    }
   }
 
   T* operator->() const { return &*p; }