export-bundle can generate an invalid bundle when waiting for machine: invalid placement syntax ""

[Nobuto Murata]

$ juju version
2.9.37-ubuntu-amd64

I know it's a corner case, but export-bundle can generate an invalid bundle in the following case.

How to reproduce:
1. add-unit and confirm an application has an unit in "waiting allocating" status
2. juju export-bundle > exported.yaml
3. juju add-model another-model
4. juju deploy --dry-run ./exported.yaml

$ juju deploy --dry-run ./exported.yaml
ERROR cannot deploy bundle: the provided bundle has the following errors:
invalid placement syntax ""

[juju status]

Unit Workload Agent Machine Public address Ports Message
ceph-mon/0 active idle 0/lxd/0 192.168.151.117 Unit is ready and clustered
ceph-mon/1 active idle 1/lxd/1 192.168.151.116 Unit is ready and clustered
ceph-mon/2* active idle 2/lxd/0 192.168.151.129 Unit is ready and clustered
ceph-mon/3 waiting allocating waiting for machine

[exported.yaml]

  ceph-mon:
    charm: ceph-mon
    channel: quincy/stable
    revision: 109
    num_units: 4
    to:
    - lxd:0
    - lxd:1
    - lxd:2
    - ""