You can model loot farming as a binomial distribution with your desired item's drop rate as chance of success, and the remainder of probabilities as the chance of failure. I use this binomial calculator to figure out the probabilities. Let's plug in your numbers and interpret the results:
Probability of success: 0.51 (51%)
Number of trials: 7
Number of successes: 1
P(X=x) = 4.94%, you had a roughly 1/20 chance to get it EXACTLY once. Doesn't paint a complete picture though.
P(X<x) = 0.68%, you had less than 1% chance to get less than one. This is where you are... so yes, you were unlucky. But people get 0.5% mods all the time! Your situation is not that weird.
P(X<=x) is just the previous two added up, not that interesting
P(X > x) = 94.38%, you had that much odds of getting MORE than 1 mod
P(X >= x) = 99.32%, and this is the inverse of P(X<x), says the same thing
Disclaimer: I barely passed my Stats & Probabilities classes, so there might be some errors in how I think about randomness.
Farming is a mental game, and when you hit these strings of bad luck, one thing I like to do (or liked to do, I suppose, since I own all mods now) is to turn it into a game of "how big of a streak am I going to get". Another thing I like to do is to use the calculator to set myself a probabilistic goal. You had a 99.32% chance to get at least one... Try to get to 99.5%! Turns out only a single extra run brings you to 99.67% chance of getting at least one mod. It quickly converges to 99.9999999% after a certain point...
Mark down the times you did your runs and who you were with, and if you do 20 runs without getting the mod, in my opinion you would be completely justified in submitting a support ticket to DE giving them your farming log and this probability calculation (though I can't tell you what their response might be). Personally though, I'd be surprised if you didn't get it in your next 3 runs. Or 4, or 5... who knows? It's RNG... All I can say is that you're on the extreme verge of getting one. If Nekros still works for that, maybe try bringing one?
If you consider that the player base is 40 million players, there's bound to be a lot of freak improbabilities. I feel like that would arbitrarily make random players unable to obtain random things for themselves... Which is why I personally believe there should be a probabilistic cap of sort, an internal counter where if you try and get something, it starts counting your runs, and after X runs where you would've had 99.5% chance of getting it, you get it automatically. And after you get 1, or 2, or some number depending on the item, it stops counting and RNG works naturally (i.e. you're not entitled to getting a Pistol Amp every 10 runs like clockwork forever, to use this example)
Good luck, Tenno!