[clang][CodeGen] Check initializer of zero-size fields for nullptr

llvm#96422 changed treats empty
records as zero-sized for the purpose of layout. In `C`, empty fields
were never considered `isZeroSize`, so we would never have tried to
call `Init->hasSideEffects` on them. But since llvm#96422
we can get here when compiling `C`, but the `Init` need to exist. This
patch adds a null-check to account for this situtation.

Author: Michael137

clang/lib/CodeGen/CGExprConstant.cpp

@@ -738,7 +738,7 @@ bool ConstStructBuilder::Build(const InitListExpr *ILE, bool AllowOverwrite) {
     // Zero-sized fields are not emitted, but their initializers may still
     // prevent emission of this struct as a constant.
     if (isEmptyFieldForLayout(CGM.getContext(), Field)) {
-      if (Init->HasSideEffects(CGM.getContext()))
+      if (Init && Init->HasSideEffects(CGM.getContext()))
         return false;
       continue;
     }

clang/test/CodeGenCXX/union-empty-field-init.c

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 %s -emit-llvm -triple x86_64-linux-gnu -o - | FileCheck %s --check-prefixes=CHECK
+// RUN: %clang_cc1 -x c++ %s -emit-llvm -triple x86_64-linux-gnu -o - | FileCheck %s --check-prefixes=CHECK-CXX
+
+union Foo {
+  struct Empty {} val;
+};
+
+union Foo foo = {};
+
+// CHECK: @foo = {{.*}}global %union.Foo undef, align 1
+// CHECK-CXX: @foo = {{.*}}global %union.Foo undef, align 1