[utest] fix libc issue

components/utilities/utest/utest_assert.h

@@ -50,8 +50,8 @@ void utest_assert_buf(const char *a, const char *b, rt_size_t sz, rt_bool_t equa
 */
 #define uassert_true(value)      __utest_assert(value, "(" #value ") is false")
 #define uassert_false(value)     __utest_assert(!(value), "(" #value ") is true")
-#define uassert_null(value)      __utest_assert((const char *)(value) == NULL, "(" #value ") is not null")
-#define uassert_not_null(value)  __utest_assert((const char *)(value) != NULL, "(" #value ") is null")
+#define uassert_null(value)      __utest_assert((const char *)(value) == RT_NULL, "(" #value ") is not null")
+#define uassert_not_null(value)  __utest_assert((const char *)(value) != RT_NULL, "(" #value ") is null")
 
 #define uassert_int_equal(a, b)      __utest_assert((a) == (b), "(" #a ") not equal to (" #b ")")
 #define uassert_int_not_equal(a, b)  __utest_assert((a) != (b), "(" #a ") equal to (" #b ")")