transient-suffix-object: Restore fallback behavior during setup

In [1: 1b7f9be] we started to signal an error if there are multiple
bindings for the command and we cannot determine the correct suffix
object based on the key binding.

When COMMAND is non-nil, then this function is not being called on
behalf of `this-command'.  In that case `this-command-keys' is also
meaningless, and when COMMAND has multiple bindings, then COMMAND
cannot be mapped to a single suffix object.  Go back to returning
the first object.

Closes #325.

1: 2024-11-02 1b7f9be
   transient-suffix-object: Drop faulty fallback behavior

lisp/transient.el

@@ -1709,7 +1709,12 @@ probably use this instead:
                         (listify-key-sequence (this-command-keys))))
                suffixes)
             (car suffixes))
-          (error "BUG: Cannot determine suffix object"))))
+          ;; COMMAND is only provided if `this-command' is meaningless, in
+          ;; which case `this-command-keys' is also meaningless, making it
+          ;; impossible to disambiguate redundant bindings.
+          (if command
+              (car suffixes)
+            (error "BUG: Cannot determine suffix object")))))
    ((and-let* ((obj (transient--suffix-prototype (or command this-command)))
                (obj (clone obj)))
       (progn ; work around debbugs#31840