Remove duplicated display() class method from RendererGroup, fixing bugs

This is actually achieved by replacing the version in RenderGroup's parent
class AnalyzerRenderer (which was buggier, missing one esc_attr() and an
"\n" in bare mode) with the one from RendererGroup, and replacing one
is_null() check for the contents of a member variable by an isset() (which
extends the check to the existence of that member).