Overriding an existing rule with no action leads to NPE

[Andres Almiray]

* Exception is:
java.lang.NullPointerException
        at org.kordamp.gradle.plugin.enforcer.internal.AbstractEnforcerRuleInvoker$_createAndInvokeRules_closure1.doCall(AbstractEnforcerRuleInvoker.groovy:91)
        at org.kordamp.gradle.plugin.enforcer.internal.AbstractEnforcerRuleInvoker$_createAndInvokeRules_closure1.call(AbstractEnforcerRuleInvoker.groovy)
        at org.codehaus.groovy.runtime.DefaultGroovyMethods.each(DefaultGroovyMethods.java:2330)
        at org.codehaus.groovy.runtime.DefaultGroovyMethods.each(DefaultGroovyMethods.java:2315)
        at org.codehaus.groovy.runtime.DefaultGroovyMethods.each(DefaultGroovyMethods.java:2356)
        at org.kordamp.gradle.plugin.enforcer.internal.AbstractEnforcerRuleInvoker.createAndInvokeRules(AbstractEnforcerRuleInvoker.groovy:91)
        at org.kordamp.gradle.plugin.enforcer.internal.BuildEnforcerRuleInvoker.settingsEvaluated(BuildEnforcerRuleInvoker.groovy:79)
        at sun.reflect.GeneratedMethodAccessor11.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.gradle.configuration.internal.DefaultListenerBuildOperationDecorator$BuildOperationEmittingInvocationHandler.invoke(DefaultListenerBuildOperationDecorator.java:226)
        at com.sun.proxy.$Proxy123.settingsEvaluated(Unknown Source)
        at sun.reflect.GeneratedMethodAccessor11.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:36)