Simplify pprLHsContext

This removes an ad hoc special case for empty LHsContexts in pprLHsContext, fixing #20011 (closed). To avoid regressions in pretty-printing data types and classes constructed via TH, we now apply a heuristic where we convert empty datatype contexts and superclasses to a Nothing (rather than Just an empty context). This will, for instance, avoid pretty-printing every TH-constructed data type as data () => Blah ....