Specifying a non-existing variable in "unless" attribute causes a NullPointerException

Steps to reproduce

1. run a zul with the code below:

   label unless="${notExistingVar}" value="Label Text"/>
   

actual result

java.lang.NullPointerException
at org.zkoss.zk.ui.util.ConditionImpl.isEffective(ConditionImpl.java:92)
at org.zkoss.zk.ui.metainfo.BranchInfo.isEffective(BranchInfo.java:104)
at org.zkoss.zk.ui.metainfo.ComponentInfo.isEffective(ComponentInfo.java:60)
at org.zkoss.zk.ui.impl.UiEngineImpl.isEffective(UiEngineImpl.java:1087)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:747)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:822)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:771)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate(UiEngineImpl.java:709)
at org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage0(UiEngineImpl.java:452)
at org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage(UiEngineImpl.java:358)
at org.zkoss.zk.ui.http.DHtmlLayoutServlet.process(DHtmlLayoutServlet.java:217)
at org.zkoss.zk.ui.http.DHtmlLayoutServlet.doGet(DHtmlLayoutServlet.java:136)

Debug information

• This issue doesn't happen in ZK 7.0.6
• The attribute if also has the same issue.

Workaround

• use shadow element with boolean operators like <if test="${notExistingVar ne null}"> instead.
• use stricter syntax like unless = $ {var ne null and var)", if the var is a boolean.
• add the property below in zk.xml

  <system-property>
      <name>org.zkoss.zel.impl.parser.COERCE_TO_ZERO</name>
      <value>true</value>
  </system-property>