Protect against "no PR identified"

Without this, if the PR branch has been deleted, you see this ("Finishing PR<>"):

> pr_finish()
ℹ Finishing PR <>
! Local branch "r-wd-use-forward-slash" has no associated remote branch.
ℹ If we delete "r-wd-use-forward-slash", any work that exists only on this branch may be hard for you to recover.

Author: jennybc

R/pr.R

@@ -574,10 +574,13 @@ pr_clean <- function(
   } else {
     pr <- pr_get(number = number, tr = tr)
   }
-  ing <- switch(mode, finish = "Finishing", forget = "Forgetting")
-  ui_bullets(c(
-    "i" = "{ing} PR {.href [{pr$pr_string}]({pr$pr_html_url})}"
-  ))
+
+  if (!is.null(pr)) {
+    ing <- switch(mode, finish = "Finishing", forget = "Forgetting")
+    ui_bullets(c(
+      "i" = "{ing} PR {.href [{pr$pr_string}]({pr$pr_html_url})}"
+    ))
+  }
 
   pr_local_branch <- if (is.null(pr)) git_branch() else pr$pr_local_branch