From the findbugs-maven-plugin usage documentation states that the visitors
/omitVisitors
options both specify a comma-separated list of bug detectors which should be run/not run. The bug detectors are specified by their class names, without any package qualification.
The class which checks for the redundant null check of non-null value is, as far as I can tell, FindNullDeref. However it does a number of other checks as well, so you would also be turning them off. Not sure if it is possible to turn off just that one check that is bothering you.