433 MHz module antenna ground
I can't disclose the full circuit because of an NDA but here is the antenna input section for a similar receiver. This one includes a capacitor in series with the antenna connection, which presumably yours doesn't if you're able to measure a DC resistance:



So when you're measuring the DC resistance the inductor in the parallel LC circuit will appear as a short to ground, but of course that won't be the case at the RF frequencies of interest. So don't try to remove it because it'll compromise receiver performance and you shouldn't need to take it into account when calculating the antenna length.

The PCB and pin lengths may have a bit of an effect of the optimal antenna length but if you're keen enough probably the easiest way to tune it without any test gear is to start with the antenna a bit longer than theoretically required and trim it down until you get the maximum range. That's assuming you're using wire for the antenna that can easily be replaced if you end up making it too short before finding the best length.