Drop unnecessary NULL check

The variable `key` is checked in the beginning of the function to be
non-NULL.
In the while loop it gets modified via `key++;` making it impossible to
be NULL after the loop.

check_after_deref: Null-checking key suggests that it may be null, but it has already been dereferenced on all paths leading to the check.

Found by Coverity

src/persistence.c

@@ -349,7 +349,7 @@ migrate_unique_key (char *key) {
     key++;
     delims++;
   }
-  if (key && delims == 2) {
+  if (delims == 2) {
     sprintf (agent_hex, "%" PRIx32, djb2 ((unsigned char *) key));
     append_str (&nkey, agent_hex);
   }