Avoid a nullptr string in dependency_graph_test

Don't define a string member to default to =nullptr, as that would
crash if it was ever used, and wouldn't compile in C++23.

Bug: chromium:1302724
Change-Id: I2ce146183e2fe4427b2d0c5ed8b4c42bff9429e2
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/82660
Auto-Submit: Tomasz Śniatowski <tsniatowski@vewd.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@chromium.org>
Commit-Queue: Ben Clayton <bclayton@chromium.org>

src/tint/resolver/dependency_graph_test.cc

@@ -1231,7 +1231,7 @@ TEST_F(ResolverDependencyGraphTraversalTest, SymbolsReached) {
   struct SymbolUse {
     const ast::Node* decl = nullptr;
     const ast::Node* use = nullptr;
-    std::string where = nullptr;
+    std::string where;
   };
 
   std::vector<SymbolUse> symbol_uses;