Be safer when handling types during completion

* jupyter-client.el (jupyter-completion-construct-candidates): Don't
allow a potential call to `max` with zero arguments.  Consider the
potential that there may not be types for all matches. In this case,
the type information displayed will be wrong for some matches and not
appear for others.  In the type information there is available the
actual match that it is supposed to be associated with which could
be used to assign the type information to the exact match.

Author: nnicandro

jupyter-client.el

@@ -1519,10 +1519,16 @@ supplied by the kernel."
              (aset matches i (car (last (split-string match "\\.")))))))
         (when buf (kill-buffer buf))))
     ;; When a type is supplied add it as an annotation
-    (when-let* ((types (plist-get metadata :_jupyter_types_experimental)))
-      (let ((max-len (apply #'max (mapcar #'length matches))))
+    (when-let* ((types (plist-get metadata :_jupyter_types_experimental))
+                (lengths (mapcar #'length matches)))
+      (let ((max-len (apply #'max lengths)))
         (cl-loop
-         for i from 0 below (length matches)
+         for i from 0 below
+         ;; For safety, ensure the lengths are the same which is
+         ;; usually the case, but sometimes a kernel may not return
+         ;; lists of the same length.  Seen for example in IJulia.
+         (min (length matches) (length types))
+         ;; These are typically in the same order.
          for match = (aref matches i)
          for meta = (aref types i)
          do (let* ((prefix (make-string (1+ (- max-len (length match))) ?\s))
@@ -1532,6 +1538,9 @@ supplied by the kernel."
                0 1 'docsig
                (concat (get-text-property 0 'docsig match) sig) match)
               (put-text-property 0 1 'annot annot match)))))
+    ;; FIXME To get rid of this conversion use `json-array-type', be
+    ;; sure to change places where it is assumed that there are
+    ;; vectors.
     (append matches nil)))
 
 ;;;;; Completion at point interface