[mono][llvm] Fix the emission of nonnull metadata on loads. (#86071)

Fixes llvm verification errors in newer llvm versions:
```
nonnull metadata must be empty
   %49 = load ptr, ptr @aotconst_ldstr_21, align 8, !nonnull !1
```

src/mono/mono/mini/mini-llvm.c

@@ -2419,14 +2419,12 @@ set_metadata_flag (LLVMValueRef v, const char *flag_name)
 static void
 set_nonnull_load_flag (LLVMValueRef v)
 {
-	LLVMValueRef md_arg;
 	int md_kind;
 	const char *flag_name;
 
 	flag_name = "nonnull";
 	md_kind = LLVMGetMDKindID (flag_name, (unsigned int)strlen (flag_name));
-	md_arg = md_string ("<index>");
-	LLVMSetMetadata (v, md_kind, LLVMMDNode (&md_arg, 1));
+	LLVMSetMetadata (v, md_kind, LLVMMDNode (NULL, 0));
 }
 
 static void