(edit: I am unable to reproduce this effect anymore, must have been an unlucky glitch)
/execute unless entity @p[nbt={Inventory:[{id:"minecraft:snowball"}]}]
I put this into a command block, and set that command block to "repeat" and "always active." I then put a comparator next to that command block to activate a separate command block that gives me a snowball. This as a whole gave me infinite snowballs without filling my inventory entirely of snowballs. The downside to this is that it can somehow change the view of the whole world. You can see through blocks, some blocks turn red and blue, and sometimes there's this shadow a few blocks ahead of the player, almost as if there's a blindness effect. To fix this you just exit out of Minecraft completely. If the command activates again then the weirdness will also activate again, so break the command block before you leave.
Can you attach a screenshot to show what you mean?
Also, a few recommendations:
use a chained command block that is set to conditional. This will replace the need for a comparator
use the /execute unless data entity command to check for data.