Placement rule can't create if some store have matched the rule.

## Bug Report
## Enhancement Task

### What did you do?
And I have one store
```
store 5:
data=cold
exclusive=exclusive
```

Then I create rule:
```
create placement policy t1 contraints="data=cold"
```

later, I want to cancel the exclusive label from the stores, and let some region move in .

### What did you expect to see?
This rule can be create successful .
### What did you see instead?
```
rule 'table_rule_xxxx' from rule group 'TiDB_DDL_244' can not match any store
```
some related code :
https://github.com/tikv/pd/blob/a1c343be47e72d7d5097b0f25df104f14579c64f/server/schedule/placement/rule_manager.go#L213-L218
### What version of PD are you using (`pd-server -V`)?
master