Fix NPE when attempting to enable NextFlow in the root container

[labkey-jeckels]

Rationale

java.lang.NullPointerException: Cannot invoke "org.labkey.api.data.Container.getId()" because "c" is null
org.labkey.api.data.PropertyCache.getCacheKey(PropertyCache.java:59)
org.labkey.api.data.PropertyCache.getProperties(PropertyCache.java:37)
org.labkey.api.data.AbstractPropertyStore.getProperties(AbstractPropertyStore.java:58)
org.labkey.api.data.AbstractPropertyStore.getProperties(AbstractPropertyStore.java:73)
org.labkey.api.data.PropertyManager.getProperties(PropertyManager.java:96)
org.labkey.nextflow.NextFlowManager.isEnabled(NextFlowManager.java:104)
org.labkey.nextflow.NextFlowController$BeginAction.getView(NextFlowController.java:201)
org.labkey.nextflow.NextFlowController$BeginAction.getView(NextFlowController.java:186)
org.labkey.api.action.FormViewAction.handleRequest(FormViewAction.java:110)

Changes

• Don't try to get the inherited value when invoked at the root