Initialize Unit.cpp_ast to null by default.

It's not initialized everywhere and no warning is given.
https://github.com/carbon-language/carbon-lang/blob/43b9969058b3de2e0b05896ef8e4489c1dc85cb6/toolchain/language_server/context.cpp#L146

toolchain/check/check.h

@@ -29,7 +29,7 @@ struct Unit {
   SemIR::File* sem_ir;
 
   // The Clang AST owned by `CompileSubcommand`.
-  std::unique_ptr<clang::ASTUnit>* cpp_ast;
+  std::unique_ptr<clang::ASTUnit>* cpp_ast = nullptr;
 };
 
 // Checks a group of parse trees. This will use imports to decide the order of