Fix SDL deptracer crash on link property computables
Computable link properties will usually have no parent link context when
traced, and they're new objects and cannot be a dependency.
@@ -244,6 +244,11 @@ def trace_Path(node: qlast.Path, *,
return
if step.type == 'property':
+ if ptr is None:
+ # This is either a computable def, or
+ # unknown link, bail.
+ return
+
lprop = ptr.getptr(ctx.schema, step.ptr.name)
if lprop is None:
# Invalid link property reference, bail.
