minor URDF fixes

Author: baggepinnen

src/Multibody.jl

@@ -121,7 +121,7 @@ urdf2multibody(joinpath(dirname(pathof(Multibody)), "..", "test/doublependulum.u
 `render_fixed = false`: Whether or not to render "fixed" joints. These joints aren't actually joints (no degrees of freedom), they are translated to FixedTranslation or FixedRotation components.
 """
 function urdf2multibody end
-export urdf2multibody, URDFRevolute, URDFPrismatic
+export urdf2multibody, URDFRevolute, URDFPrismatic, NullJoint
 
 const t = let
     (@independent_variables t)[1]

src/joints.jl

@@ -807,9 +807,9 @@ end
 
 end
 
-@component function URDFRevolute(; name, r, R, axisflange = false, kwargs...)
+@component function URDFRevolute(; name, r, R=I(3), axisflange = false, kwargs...)
     if R == I(3) && r == zeros(3)
-        return j
+        return Revolute(; axisflange, name, kwargs...)
     end
 
     systems = @named begin
@@ -848,7 +848,7 @@ end
 
 @component function URDFPrismatic(; name, r, R, axisflange = false, kwargs...)
     if R == I(3) && r == zeros(3)
-        return j
+        return Prismatic(; axisflange, name, kwargs...)
     end
 
     systems = @named begin