Most (all I know) '51 derivatives come up with random contents of the (internal) RAM. If you look at e.g. keils startup.a51 you will see that it initializes all RAM (internal and external) to zero.
This is standard practice and I strongly recommend that you follow it, even if a particular derivative happens to come up zero as this may not be the result when the chip is revised. No '51 I know of speify that the RAM is initialized at power up.

Again: testing does not prove that something works.

Have fun,

Erik