Nav map macro should correctly handle requests to see maps for labels that do not exist.

At the moment, we see "Error formatting macro: navmap: java.lang.NullPointerException" on the rendered page and get the following in the logs:

2005-12-15 12:14:14,240 ERROR [renderer.v2.components.MacroRendererComponent] Unexpected error formatting macro: navmap
java.lang.NullPointerException
at com.atlassian.confluence.labels.DefaultLabelManager.getCurrentContentForLabel(DefaultLabelManager.java:345)
at com.atlassian.confluence.labels.DefaultLabelManager.getContent(DefaultLabelManager.java:501)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at org.springframework.aop.framework.AopProxyUtils.invokeJoinpointUsingReflection(AopProxyUtils.java:61)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:149)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:116)
at com.atlassian.confluence.util.profiling.SpringProfilingInterceptor.invoke(SpringProfilingInterceptor.java:16)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:138)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:56)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:138)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:152)
at $Proxy6.getContent(Unknown Source)
at com.atlassian.confluence.renderer.v2.macros.NavigationMapMacro.execute(NavigationMapMacro.java:49)
at com.atlassian.renderer.v2.macro.ResourceAwareMacroDecorator.execute(ResourceAwareMacroDecorator.java:45)
at com.atlassian.renderer.v2.components.MacroRendererComponent.processMacro(MacroRendererComponent.java:302)