NPE when update ParamFlowRule before the resource has been requested once

[MrCoderYu]

Type: bug report

Describe what happened (or what feature you want)

when update ParamFlowRule with ParamFlowRuleManager.loadRules()
if one of the resource has not bean requested before(there is no metric found),then NPE occurs
the code is here:[ParameterMetricStorage.getParamMetricForResource(resource).clearForRule(rule);]

Describe what you expected to happen

judge if the metric is null or not...

How to reproduce it (as minimally and precisely as possible)

1. init a api resource with ParamRuleManager
2. update that before that has been requested

Tell us your environment

1.8.0

Anything else we need to know?

[brotherlu-xcq]

we just need to judge if the resource has metric before we clear it to avoid the NPE when we update the param flow rule. below is place which the NPE happened。

@i want to resolve it@
@sczyh30

[sczyh30]

Contributions are welcomed!