Is a research thesis (report) with zero contribution to human knowledge acceptable?
I assume that by negative results, you mean non-significant results.

"It is an expectation that the PhD would make an original contribution and/or advance knowledge in a given field." Yes, this is true. And "X doesn't work" is a contribution to the field.

(To use an example from my area). Health practitioners are constantly dreaming up things that might work to treat various ailments (illnessess), and using them. The job of health researchers is to find out which ones work - and most of them don't work.

We used to joke that our role as health care researchers was to say no. "Nope. That doesn't work. Don't do it. No, that one's not effective either. No, don't use that. No. No. No."

Pressure for positive results just means you tweak models and data until you find them - using 'researcher degrees of freedom' (see http://pss.sagepub.com/content/22/11/1359). Your results are therefore significant, but worthless.

My PhD thesis was trying to demonstrate the nature of the relationship between stress and psoriasis symptoms (many people say "stress worsens psoriasis" - it's taken as a given truth, but it's never been empirically demonstrated). I was trying to answer things like what kind of stress, how long does it take, does it differ between people? I never found any evidence that stress did worsen psoriasis. Nor that psoriasis worsened stress (or any other psychological symptom).  

A PhD is an educational process. One should demonstrate that one has learned. The most important thing about a PhD is showing what you know, what you have learned, and what you understand. If anyone gets to the end of a PhD and says "Well, those results were all positive, just as I expected", they've learned little. At the end of your PhD (or any research project) you should want to start again, and this time do it properly.