fix: Always allow ps:scale proc=0

With the introduciton of procfile-util, scaling a non-existent process to 0 was disabled. This brings back that functionality, allowing users to scale all processes to 0. This is especially useful when a user has accidentally scaled a non-existent process up, causing deploys to fail without any way to revert outside of manually editing files.

Closes #3428

tests/unit/10_ps-herokuish.bats

@@ -109,11 +109,11 @@ teardown() {
     assert_success
   done
 
-  run /bin/bash -c "dokku ps:scale $TEST_APP web=0 worker=0"
+  run /bin/bash -c "dokku ps:scale $TEST_APP web=0 worker=0 invalid-process=0"
   echo "output: $output"
   echo "status: $status"
   assert_success
-  for PROC_TYPE in web worker; do
+  for PROC_TYPE in web worker invalid-process; do
     CIDS=""
     shopt -s nullglob
     for CID_FILE in $DOKKU_ROOT/$TEST_APP/CONTAINER.$PROC_TYPE.*; do