fix compiler segfault on null ?? x

closes #390

src/codegen.cpp

@@ -4224,6 +4224,7 @@ static void define_builtin_types(CodeGen *g) {
     {
         TypeTableEntry *entry = new_type_table_entry(TypeTableEntryIdNullLit);
         buf_init_from_str(&entry->name, "(null)");
+        entry->zero_bits = true;
         g->builtin_types.entry_null = entry;
     }
     {

test/cases/null.zig

@@ -143,3 +143,8 @@ test "unwrap nullable which is field of global var" {
         unreachable;
     }
 }
+
+test "null with default unwrap" {
+    const x: i32 = null ?? 1;
+    assert(x == 1);
+}