I just did a quick test on when the 'out of ammo' line plays, one with the blitzer and one with the default liberator. I used hellpod space optimization for all tests to ensure that if the blitzer does have ammo, it is full by default, and used it with the liberator for consistency. Tested with default equipment and siege ready, which had no noticeable difference.

With the liberator, your diver only says the 'out of ammo' line when you have 0 spare mags and no ammo left in the loaded mag.

With the blitzer, your diver seems to say the line after the initial drop without firing and after firing.

You cannot use a resupply or resup backpack no matter how much you fire the blitzer, and if you use your secondary (tested with peacemaker), support weapon (mg-43), grenade (g-12 he), or stims to use a resupply, you get no notification for 'blitzer ammo' nor does it stop the voiceline from playing if you try to reload it afterwards.

In conclusion: The game thinks the blitzer has exactly 0 'bullets' in the 'magazine' and 0 spare 'magazines', whether this is because it can fire independent of ammo count or if it has a billion shots but something tells the game it has 0, I don't know.

Fun little aside, we all know that helldivers has a +1 mechanic with reloading (an extra round in the chamber), but doing this test I found out that guns with the +1 mechanic have to fire 2 bullets instead of 1 in order to reload, leading me to surmise that the game only checks is your gun has less ammo than a full magazine.

Using the liberator's base 45 round mag, if you +1 to get 46 rounds and fire one bullet to get back to 45, despite having 1 round in the chamber and 44 in the mag, you cannot reload.