patch 8.2.0634: crash with null partial and blob

Problem:    Crash with null partial and blob.
Solution:   Check for NULL pointer.  Add more tests. (Yegappan Lakshmanan,
            closes #5984)
diff --git a/src/eval.c b/src/eval.c
index 659588f..4bd45e9 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3682,7 +3682,9 @@
 {
     if (pt->pt_name != NULL)
 	return pt->pt_name;
-    return pt->pt_func->uf_name;
+    if (pt->pt_func != NULL)
+	return pt->pt_func->uf_name;
+    return (char_u *)"";
 }
 
     static void