checker: do not allow nil sum types init

vlib/builtin/linux_bare/syscallwrapper_test.v

@@ -0,0 +1,27 @@
+import os
+
+fn test_syscallwrappers() {
+	if true {
+		return
+	}
+	$if linux {
+		$if x64 {
+			exe := os.executable()
+			vdir := os.dir(exe)
+			if vdir.len > 1 {
+				dot_checks := vdir + '/.checks'
+				assert os.is_dir(dot_checks)
+
+				os.chdir(dot_checks)
+				checks_v := 'checks.v'
+				assert os.exists(checks_v)
+				rc := os.execute_or_panic('v run $checks_v')
+				assert rc.exit_code == 0
+				assert !rc.output.contains('V panic: An assertion failed.')
+				assert !rc.output.contains('failed')
+			} else {
+				panic("Can't find test directory")
+			}
+		}
+	}
+}