Hello and welcome
Some people with MS do not have lesions on their MRI scans, especially in the early years, although a really good MRI scanner and high resolution images might show something up that standard MRI would miss. Lumbar puncture results (spinal fluid) are also variable: about 10% of people get a negative result despite definitely having MS, but then it is also possible to have a positive result and not have MS.
Because there is no test that is always positive in MS, it is possible to diagnose patients completely from their symptoms, history and clinical exam, assuming all other possibilities have been ruled out. And so, yes, there are people who have been diagnosed with only a positive LP and there are even people who have been diagnosed with all negative tests. It’s quite unusual though.
I think, if I were you, I would be asking what other tests would help to work out what is going on. For example, have you had evoked potentials or blood tests for all the possible mimics? Ultimately though, time will tell - and if you have the support of your neuro and GP in the meantime, your care will not be very different to someone with a formal diagnosis.
I hope the steroids help, and that you get some answers soon.